Preheat the oven to 350°F. Spray the bottom and sides of a 13x9 inch pan with baking spray with flour.
In a large bowl, beat the cake mix, flour, water, oil and eggs with an electric mixer on LOW speed for 30 seconds. Beat on MEDIUM speed for 2 minutes, scraping the bowl occasionally. Stir in 1/2 cup of the toffee bits. Pour into the pan.
Reserve 1/2 cup of the dulce de leche. Spoon the remaining dulce de leche by teaspoonfuls onto the batter.
Bake for 30 to 40 minutes or until a toothpick inserted in the center comes out clean. Cool for 5 minutes. Drop the reserved dulce de leche by spoonfuls over the top of the cake and spread evenly. Sprinkle with the remaining toffee bits. Cool about 1 hour before serving. Top each serving with whipped cream and caramel topping.
* Dulce de leche, a caramelized condensed milk, can be found near the sweetened condensed milk in most grocery stores.
** You can substitute crushed chocolate-covered English toffee candy bars for the toffee bits.