Preheat the oven to 350° F, and line a cupcake pan with 4 liners.
In a small bowl, stir together the melted butter and sugar with a whisk.
Add the egg and nutmeg, and whisk until combined.
Sprinkle the flour and baking powder over the butter and sugar mixture, and whisk together until just combined—do not overmix.
Divide the batter between the 4 cupcake liners, and bake for 16-18 minutes, until the cupcakes are domed and a toothpick inserted comes out with only moist crumbs clinging to it.
Let the cupcakes cool completely.
Meanwhile, beat together the softened butter and powdered sugar for the frosting. Add enough whiskey to make the frosting spreadable.
Frost the cupcakes, decorate with dulce de leche and serve.