Cake

1. Preheat oven to 350 degrees F (180 degrees C). Grease two 9-inch cake pans and set aside.
2. In a large mixing bowl, cream together the butter and sugars until light and fluffy. Add the eggs one at a time, beating well after each addition. Beat in the sour cream, vegetable oil, dulce de leche, and vanilla extract. Gradually add baking powder, baking soda, salt, and half of the cake flour. Stir in the milk, and add the remaining cake flour, mixing until batter is uniform and smooth.
3. Divide the batter evenly between the prepared baking pans and bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean. Remove from the oven, transfer cakes to a cooling rack, and allow to cool to room temperature before frosting or serving.

Frosting

1. Wipe a large bowl with a paper towel soaked in a small amount of lemon juice or vinegar to remove traces of grease. Add egg whites and sugar and, over a double boiler, whisk constantly over hot water until the sugar dissolves. When rubbed between your fingers, the egg whites should feel hot and smooth (approximately a temperature of 140 degrees F/60 degrees C). This will generally take 3-5 minutes.
2. Using a mixer, whip the egg whites until thick, glossy peaks form. This may take anywhere from 8-10 minutes. The bottom of the bowl should feel neutral to the touch before moving on to the next step.
3. Place the mixer on low speed and add the butter in small pieces, one at a time, mixing until smooth. The frosting should reach a silky texture. Beat in the vanilla. If the butter is too warm and the frosting is too runny, place in the refrigerator for approximately 15 minutes to firm up the butter before mixing again. If the mixture curdles, continue mixing and it will come back together. Stir in 1/3 cup of the dulce de leche sauce.
4. Place the bottom cake layer on a serving plate. Place approximately 2 cups of frosting on the cake and smooth evenly. Place the second layer on top and smooth frosting onto the top and sides of the cake. Using a spoon, dot the remaining dulce de leche sauce on the outside of the cake and smooth to create the ombré look.