Melt butter and chocolate squares in a heavy saucepan over low heat.
Beat eggs and sugar at medium-high speed with an electric mixer 8 minutes. Gradually add chocolate mixture, beating at low speed until blended. Gradually add flour, beating until blended. Stir in chocolate morsels. Pour into a lightly greased 13- x 9-inch pan.
Bake at 350° for 30 to 35 minutes. Cool on a wire rack. Cut into squares. Garnish, if desired.