Heat oven to 350°F (or 325°F for dark or nonstick pan). Spray bottom and sides of 13x9-inch pan with baking spray with flour.
In large bowl, beat cake mix, flour, water, oil and eggs with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Stir in 1/2 cup of the toffee bits. Pour into pan.
Reserve 1/2 cup dulce de leche. Spoon remaining dulce de leche by teaspoonfuls onto batter.
Bake 30 to 40 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes. Drop reserved dulce de leche by spoonfuls over top of cake and spread evenly. Sprinkle with remaining toffee bits. Cool about 1 hour before serving. Top each serving with whipped cream and caramel topping.