Remove label from sweetened condensed milk and place on its side in a pot. Extremely important: Cover the can with water to at least 2 inches above the can, making sure the water level always covers the can during the cooking process. Bring water to a boil, cover and reduce heat to rolling boil. Cook for 2 ½ hours, again, making sure the water covers the can. (You can also place the can in a pot in the oven using the same method of covering the can with water. Cook at 425 degrees for 1 ½ hours). When done, cool the can before opening. When ready to bake, reduce oven temperature to 350 degrees. Line two regular cupcake pans with liners and lightly spray liners with cooking spray. In the bowl of an electric stand mixer, add butter and sugars, beating until fluffy, about 3 minutes. Add the eggs, vanilla and buttermilk, beating well. (Note: For homemade buttermilk, add 3 tablespoons of either vinegar or lemon juice to one cup of milk. Vigorously stir for 10 seconds and then let sit for 15 minutes to thicken.) Add 3 tablespoons of caramelized sweetened condensed milk and beat to fully incorporate. Add flour, cornstarch, baking powder, baking soda and salt and mix well. Fill each cupcake liner about two thirds full. Bake for about 18 minutes or until a toothpick comes up clean. Cupcakes should be completely cooled before frosting.