In medium microwavable bowl, microwave baking chips uncovered on Medium (50%) 4 to 5 minutes, stirring halfway through microwave time. Stir until smooth; cool 5 minutes. Stir in frosting until well blended. Place 1 cake layer bottom, cut side up, on serving plate; spread with about 3/4 cup of the frosting. Repeat with second and third cake layers. Top with reserved cake layer, cut side down; frost with remaining frosting. Store loosely covered.