That was recently answered... http://www.marketwatch.com/story/new-series-casts-bernie-mad...

> “Bernie really was a successful businessman with quite original insights into the market, and he’s continued applying his business instincts in prison,” Fishman said. “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.”

The search for "madoff swiss chocolate" will find many more articles based on the above quote.