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. dulce de leche and water until blended; reserve for later use. Blend remaining dulce de leche 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 dulce de leche mixture on HIGH 10 sec.; stir. Cool. Frost cake with COOL WHIP; drizzle with reserved dulce de leche mixture.