Preheat oven to 325˚F
Melt butter and stir in cocoa. Pour into mixing bowl; stir in sugar, eggs and vanilla. Beat well by hand. Fold in flour mix, salt and chocolate chips. Add chopped nuts if desired. Pour into greased 8 x 8-inch pan.
Bake for 30 to 35 minutes or until knife inserted in center comes out clean. Cool completely before cutting.