Prepare cake batter and bake as directed on package for 2 (9-inch) round cakes. Cool 10 min.; remove to wire racks. Cool completely.
Cut each cake layer horizontally into 2 layers; pierce with large fork at 1/2-inch intervals.
Reserve 1 tsp. drink mix. Blend milks, sour cream and remaining drink mix in blender. Stack cake layers on plate, pouring 3/4 cup milk mixture over each cake before covering with next layer. Refrigerate 1 hour.
Frost cake with COOL WHIP.
Sprinkle with reserved drink mix just before serving. Keep refrigerated.