If you have only one pan and a recipe calls for more cupcakes than your pan will make, cover and refrigerate the rest of the batter while baking the first batch. Cool the pan about 15 minutes, then bake the rest of the batter, adding 1 to 2 minutes to the bake time.
To make filling the cupcakes easier, use a decorating bag or a resealable food-storage plastic bag with one bottom corner cut off to pipe the dulce de leche.
These cupcakes are best served at room temperature. If refrigerated, let stand 15 minutes before serving.