In a 2 1/2 quart microwave safe bowl, cook chocolate pieces, uncovered, on high for 1 1/2-2 minutes or until pieces are melted and smooth, stirring half way through the cooking time.
Stir in potatoes, peanut butter and vanilla. Add powdered sugar and stir till well blended. With the back of a spoon, press the mixture into a greased 8x8x2-inch pan. Cover and chill. When firm cut into squares.