1Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 13x9-inch pan with baking spray with flour. Make and bake cake mix as directed on box for 13x9-inch pan. Cool 15 minutes. With handle of wooden spoon, poke top of warm cake every 1/2 inch.
2In medium bowl, beat filling ingredients with whisk about 2 minutes. Immediately pour over cake. Cover loosely; refrigerate about 2 hours or until chilled.
3Stir 1/4 teaspoon peppermint extract into frosting. Spread frosting over top of cake. Sprinkle with peppermint patties. Store covered in refrigerator.