Preheat oven to 325F. Cream together butter, sugar, salt and vanilla. Add cornstarch and blend well. Add flour and cocoa; blend well. Use a small ice cream scoop or use your hands to roll dough into balls; place them on a parchment-lined pan about 1" apart and use a fork to flatten the cookies. Bake for 10-15 minutes, until the bottoms just start to become golden. Yield: 2-3 dozen, depending on size.