“Berkshire needs multibillion-dollar acquisitions to move the needle,” Ms. Cool says. “So many of the people who contact us or reach out would want to sell to Berkshire, but they’re just too small.”

Ms. Cool launched an investment firm with a former colleague in 2020, called Kanbrick, that aims to focus on such companies. It has so far acquired Thirty-One Gifts, a Columbus, Ohio, company that sells tote bags, backpacks and other items through independent consultants. Kanbrick is working on investments with a home-services company and consumer brands.

Picking the right spots takes time. Before buying a business, Ms. Cool said she and her team will typically sit down with founders and aim to understand not only the fundamentals of the business, but the people working there and the strategy, hoping to answer: “What might be the challenge here, what might be the opportunity?” she said.

To get to know founders, Kanbrick also runs a three-month program for midsize companies that provides coaching and other support; so far, Ms. Cool and her colleagues have worked with 15 companies, including a large Arizona farm that sells cantaloupe and honeydew melons to big-box retailers.

The 37-year-old Ms. Cool grew up on a family farm in Kansas that shipped produce across the Midwest. She attended Harvard Business School and joined Berkshire in 2009 at age 25, initially working as Mr. Buffett’s financial assistant.

She later became chief executive of a cookware company owned by Berkshire, Pampered Chef, and along the way took on assignments within Berkshire helping struggling companies. She served as the chairman of Berkshire companies such as Benjamin Moore & Co. and Johns Manville, and sat on the board of Kraft Heinz Co. and others.

In an interview in 2019, Mr. Buffett called Ms. Cool “the fireman,” capable of helping to revive companies and of taking on any assignment.

Ms. Cool recently spoke with The Wall Street Journal from her home office in Nashville, Tenn. Here are edited excerpts:

WSJ: What was it that made you say there’s a need for this model?

Ms. Cool: Over the years, I’ve talked to a lot of founders and owners, some of whom would come to Berkshire and want to sell their companies; other people I met through organizations. What I found is that most midsize companies struggle with the same things: how to hire the right people, how to develop them, how to incentivize them, how to help them to grow, how to build a strategy. So what we did is we built a business system to help in those areas, and that really allowed us to create value with companies.

A lot of families and founders don’t want to sell to traditional private equity. They don’t want to see their business bought and sold or chopped up or their employees fired. We could provide them a longer-term home, and help them build in the right way.

WSJ: What sorts of companies are you focusing on?

Ms. Cool: We want businesses that are going to be around and successful and strong, and have some sort of moat allowing them to have above-average returns on capital. Smaller businesses that are $10 [million] to $50 million in [earnings before interest and taxes] are sort of our sweet spot in size. They’re beyond the new-growth phase, but they’re not quite very large businesses.

They tend to be family businesses, founder-owned businesses. We have a lot of conversations with families, founders who want a partner, who want a longer-term home, but don’t really think that maybe a strategic or private-equity firm is the right fit for them.

WSJ: How does your approach differ from private equity?

Ms. Cool: One is just how long we hold companies. Most private-equity firms own businesses for three or four years. If you’re going to own a company three or four years, the minute you buy it, you’re thinking about selling it, and every decision you’re making is focused on: What am I going to do to sell this business? A lot of investments you make in businesses don’t pay off in three to four years, and so I think having that longer-term horizon is super valuable.

Most people in private equity typically [have] financial backgrounds. Both my partner and I started our careers as investors but thought it was very important to go get operating experience.

I became the CEO of Pampered Chef, he became the CFO, really with that goal of: How do we actually become better at what we do? And I think having that operating experience helps us make better decisions, understand what’s possible in a business, what’s needed. And then we can relate with a founder or owner or CEO because we’ve been in their shoes and we know businesses aren’t run on spreadsheets and PowerPoints, right?

WSJ: So how do you pick your spots—and what industries are you avoiding?

Ms. Cool: There are some places that we don’t play. We don’t play in real estate. We don’t play in financials. We don’t play in biotech. There’s just spaces where we don’t have the expertise, the insight, and we’re not going to be better than someone else. Then there’s other industries where you’ve had a lot more experience and things are interesting. And so those broad industries are consumer, industrial, business services, but within those there’s hundreds of subsectors.

So we spend time looking at a lot of different ones and really saying: Do we think that this is a really great business? Do we think that it’s going to continue for 10, 15, 20 years and not be disrupted by someone else or by technology? And then, third, is it a space where we can add some valuable insights or perspective?

WSJ: You’re focusing on midsize companies. How do the challenges these companies face differ from what larger companies are experiencing?

Ms. Cool: People and culture is always—in my view—the number one issue that any company has.

How do we attract really great talent to my company? Perhaps I am based in a rural part of Minnesota or Missouri or something like that. How do I help people understand why they want to join my business that they’ve never heard of? Everyone’s heard of P&G, Coca-Cola. People haven’t heard of most midsize companies. Then, once I have them in the organization, how do I develop them? Because it’s not like I’ve got, you know, hundreds of thousands of jobs; I’ve got probably a couple hundred jobs. And so I need to get the right people, but show them a career path.

WSJ: What’s the long-term plan for Kanbrick? Do you want to go public?

Ms. Cool: We don’t have a specific outcome in terms of what we want to achieve via go public or otherwise. It really is: How do we build it in the right way? And then how do we add value to the companies, to our team, to our investors, and help support everyone in doing that?

WSJ: How have you funded Kanbrick?

Ms. Cool: It’s a combination of our capital, and then we have a select group of investors, endowments and family offices that are partners.

WSJ: It appears there are a lot of similarities between Kanbrick and Berkshire—a long-term focus, moats, you even wrote an annual letter last year like Mr. Buffett. How does it differ?

Ms. Cool: Berkshire is very successful, so being similar to Berkshire is a good thing by and large, in my mind. I’d say we differ on two dimensions. The biggest is size. We can focus on much smaller businesses that are just too small for Berkshire. That’s where I think the biggest opportunity is and why ultimately I left to start Kanbrick. The second differentiator is we’re more hands on. Berkshire famously is very hands off.

WSJ: Did Mr. Buffett give any advice that sticks with you as you’re building this firm?

Ms. Cool: It’s hard to distill it down because there’s so many lessons from my time at Berkshire and working closely with Warren over 10 years. I think just the power of long term, the power of finding high-quality businesses and the power of partnering with high-quality people. When those three things are done in the right way, you can build something really amazing.