Preheat oven to 350'F. Lightly grease cookie sheets or line with parchment paper. Beat 1 cup butter, confectioners' sugar and cocoa in large bowl until fluffy. Blend in cake flour until smooth. Shape dough into marble-size balls. (If dough is too soft to handle, cover and refrigerate until firm.) Place 2" apart on prepared cookie sheets. Press center of each ball with knuckle of finger to make indentation. Bake 10-12 minutes or until set. Remove to wire racks. Prepare Mocha Filling. While cookies are still warm, spoon about 1/2 teaspoonful filling into center of each. Makes about 5 dozen cookies. MOCHA FILLING: Melt 1 tablespoon butter and chocolate in small heavy saucepan over low heat; stir until melted. Blend in confectioners' sugar, vanilla and enough coffee to make a smooth filling.