Bernie Madoff. Shannon Stapleton/Reuters Bernie Madoff is still in business even though he is behind bars.

The convicted fraudster behind the biggest Ponzi scheme in US history hogged the market for hot chocolate in prison, according to a MarketWatch interview with Steve Fishman, a reporter who has had considerable contact with Madoff.

Madoff, 78, was sentenced in 2009 to 150 years in prison for an extensive scheme that involved doctoring his clients' account statements and returns. He is incarcerated in Butner, North Carolina.

In an interview, Fishman told MarketWatch's Ryan Vlastelica:

"Bernie really was a successful businessman with quite original insights into the market, and he's continued applying his business instincts in prison. At one point, he cornered the hot chocolate market. He bought up every package of Swiss Miss from the commissary and sold it for a profit in the prison yard. He monopolized hot chocolate! He made it so that if you wanted any, you had to go through Bernie."

Fishman is the host of a new Audible series called "Ponzi Supernova," which investigates Madoff's $65 billion fraud. It includes previously unpublished interviews with Madoff and law-enforcement agents who were involved in the case.

"I sort of rationalized that what I was doing was OK," Madoff told Eugene Soltes, a professor at Harvard Business School, in an audio interview provided to Business Insider in December.

"It wasn't going to hurt anybody," Madoff said. "It was a temporary thing, and because of the success I that I've had and the money I've made for people, I sort of felt that it would just sort of be a temporary situation and acceptable."