On medium-high speed, beat cake mix, milk, eggs, pudding and oil until fluffy (3 minutes).
Stir in mini chips. Divide batter between pans. Bake 25 minutes or until toothpick inserted into centers comes out clean. Cool 10 minutes. Transfer from pans to racks. Cool completely.
On medium-high speed, beat butter until smooth (1-2 minutes). Gradually beat in melted chips until combined. Gradually beat in confectioner's sugar, then sour cream until light and fluffy.
Place one cake layer on serving plate. Spread with 1 cup frosting. Top with remaining layer. Spread top and side with remaining frosting. Garnish with candy bars, if desired.