1. Preheat oven to 350 degrees F. Grease bottom only of 15x10x1-inch pan; lightly flour. In large bowl, beat cake mix, milk, melted butter, and 3 eggs on low speed 30 seconds, beat on medium speed 2 minutes. Pour into pan.
2. Make cookie mix as directed on package using softened butter and 1 egg. Drop dough by teaspoonfuls evenly over batter in pan.
3. Bake 35-40 minutes or until toothpick inserted in center comes out clean. Run knife around side of pan to loosen cake. Cool completely, about 1 hour. Spread frosting over top of cake. Store loosely covered at room temperature.