Beat butter and sugar at medium speed with an electric mixer until creamy. Add eggs, 1 at a time, beating until blended after each addition.
Stir together flour and cocoa; gradually add to butter mixture, beating at low speed just until blended after each addition. Stir in vanilla extract. Fold in morsels and pecans, if desired.
Pour batter into a lightly greased 13- x 9-inch pan.
Bake at 350° for 22 to 25 minutes or until a wooden pick inserted in center of cake comes out clean. Cool slightly in pan on a wire rack. Cut into squares, and serve warm.
NOTE: Semisweet chocolate morsels may be substituted for milk chocolate morsels.