PREPARATION:

1. Preheat oven to 325°F. For chocolate cookies, place half of chocolate cake mix in large bowl. Add 1/2 cup (1 stick) butter; beat with electric mixer at high speed until well blended. Add 1 egg yolk and remaining cake mix; beat just until dough forms. Beat in 1 to 2 tablespoons milk if dough is too crumbly.
2. Shape dough into 36 balls, using about 1 tablespoon dough for each cookie. Place2 inches apart on ungreased cookie sheets; flatten slightly. Bake 20 minutes or until cookies are firm. Let cookies stand on cookie sheets 5 minutes; transfer to wire racks to cool completely.
3. For vanilla cookies, place half of yellow cake mix in large bowl. Add 1/2 cup (1 stick) butter; beat with electric mixer at high speed until well blended. Add remaining egg yolk and cake mix; beat just until dough forms. Beat in 1 to 2 tablespoons milk if dough is too crumbly.
4. Shape dough into 36 balls, using about 1 tablespoon dough for each cookie. Place 2 inches apart on ungreased cookie sheets; flatten slightly. Bake 20 minutes or until cookies are firm. Let cookies stand on cookie sheets 5 minutes; transfer to wire racks to cool completely.
5. Cut remaining 1/2 cup (1 stick) butter into small pieces. Beat butter, powdered sugar, salt and 6 tablespoons milk in large bowl with electric mixer until light and fluffy. Add additional 2 tablespoons milk if necessary for more spreadable frosting. If desired, divide frosting in half. Add cocoa and 1 tablespoon milk to one half to create chocolate frosting.
6. Spread frosting on flat sides of chocolate cookies, using about 1 tablespoon per cookie. Top with vanilla cookies.