There's a recipe to do it in "Cake Decorating for Dummies"
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Dulce De Leche Filling
A popular dessert in Latin American countries, dulce de leche, which is essentially carmelized sugar in milk, has quickly found a following in cake decorating circles. This recipe is an easy way to turn dulce de leche into a fabulous filling.
Preparation time: 1 to 11â4 hours
Yield: 3 cups
2 14-ounce cans sweetened condensed milk
1 Preheat the oven to 425 degrees F.
2 Pour the milk into an 8-x-8-x-2-inch clear glass baking dish that fits into a larger, deeper cake pan. Cover the glass dish with aluminum foil.
3 Place the large cake pan on the oven rack, and set the glass dish inside it. Fill the larger pan with boiling water until the water reaches two-thirds of the way up the sides of the glass dish.
4 Bake for 1 to 11â4 hours, or until the milk turns a caramel color.
5 Use a rubber spatula to scrape the filling into a large bowl. Beat the filling on medium speed until it reaches a spreadable, smooth consistency (about 1 minute).