1Heat oven to 350°F (325°F for dark or nonstick pan). Spray 15x10x1-inch pan with cooking spray.
2Make cake mix as directed on box, using water, oil and eggs. Pour into pan. Bake 15 minutes.
3Set oven control to broil. In medium bowl, mix condensed milk, coconut and pecans; spread over warm cake. Broil on lowest oven rack about 2 minutes or until golden. Cool completely in pan on cooling rack, about 1 hour.
4In large bowl, beat butter, powdered sugar, cocoa and vanilla with electric mixer on medium speed until blended. Gradually add milk, beating until frosting is smooth and spreadable. Frost cake. Store covered in refrigerator.