Melt butter and chocolate in a heavy saucepan over medium-low heat, stirring until smooth; let cool.
Beat eggs at medium speed with an electric mixer 2 minutes; gradually add 2 cups sugar, beating well. Add melted chocolate, flour, and next 3 ingredients; beat well. Stir in peppermint candy. Pour batter into a greased and floured 13" x 9" pan.
Bake at 350° for 32 to 33 minutes. Cool completely on a wire rack. Cut into bars; sprinkle with powdered sugar.