HEAT oven to 350ºF.
PREPARE cake batter and bake as directed on package for 13x9-inch pan. Cool cake in pan 10 min. Pierce cake with large fork at 1/2-inch intervals.
MIX 2 Tbsp. caramel spread and water until blended; reserve for later use. Blend remaining caramel spread with evaporated milk and sour cream in blender until smooth. Pour over cake, re-piercing cake if necessary until all milk mixture is absorbed. Refrigerate 1 hour.
MICROWAVE reserved caramel spread mixture on HIGH 10 sec.; stir. Cool. Frost cake with COOL WHIP; drizzle with caramel. Keep refrigerated.
How to Easily Drizzle the Reserved Caramel
Use a spoon to drizzle the caramel sauce over the frosted cake. If the caramel is too thick, stir additional 1/2 tsp. water into the reserved caramel before drizzling over cake.