Cream butter in a large mixing bowl; gradually add sugar, beating well. Add melted chocolate and vanilla; beat well. Add eggs, one at a time, beating well after each addition.
Combine flour, baking powder, soda, and salt in a small mixing bowl. Stir flour mixture into creamed mixture. Divide dough in half; wrap in waxed paper, and chill 1 hour.
Roll half of dough to 1/8-inch thickness on a lightly floured surface; keep remaining dough chilled until ready to use. Cut with a 2 1/2-inch fluted or round cookie cutter. Place 2 inches apart on ungreased cookie sheets. Bake at 350° for 8 to 10 minutes. Remove from cookie sheets, and cool on wire racks.
Repeat procedure with remaining dough, cutting out center of each cookie with a 1/2- inch round cookie cutter before baking.
Prepare filling. Spread solid cookies evenly with filling. Top with remaining cookies to make sandwiches, pressing down slightly to fill cut-outs on tops of cookies.