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 5 cm 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 half 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.