Preheat oven to 325 degree F.
Spray mini muffin cups with oil; set aside.
In a small saucepan, melt butter and chocolate over low heat, stirring constantly. Let cool for 5 minutes.Stir in eggs and vanilla. Add flour and 1/4 cup M&Ms. Mix well.
Fill cups with batter and sprinkle with remaining M&Ms. Bake for 12 to 13 minutes. Remove from pan and cool completely.