Clean the mushrooms with a damp cloth. Slice thinly and cut off the stems if desired.
Heat the olive oil in the frying pan. Lay the mushrooms flat in the pan. Cook on medium heat until most of the oil is absorbed, about 3 to 5 minutes.
Add the butter or margarine. Stir in the chili powder. Cook the mushrooms on medium-high heat, stirring frequently, for 2 to 3 minutes or until browned. Serve hot.
Cooking mushrooms releases their water content, giving them extra flavor. Feel free to adapt this basic recipe to use with stronger-tasting mushrooms, such as portobello.