Caramel Sauce:

1. In a large saucepan over medium heat, melt butter and light and dark brown sugars, and milk together. Bring to a boil.
2. Let mixture boil for 3 minutes. Remove from heat and let cool.

Cupcakes:

1. Preheat oven to 350°F.
2. Line muffin tin with cupcake liners.
3. In a separate bowl, combine flour, baking powder, and salt. Whisk together.
4. In mixing bowl, beat butter and sugar over medium speed for 3 minutes. Add eggs and vanilla and continue beating until combined, scraping the sides of the bowl when necessary.
5. Add the flour and the milk, beating on low speed until ingredients are just combined.
6. Fill each liner about 3/4 full, about 1/4 cup batter.
7. Bake approximately 20 minutes until the centers are set.
8. Transfer to wire rack and let cool for 10 minutes.

Caramel Buttercream:

1. While cupcakes are cooling, make your frosting. Combine butter, powdered sugar, 3/4 cup prepared (and cooled) caramel sauce, and milk in a mixing bowl. Beat ingredients together about 2 minutes until creamy.

Assembling the Cupcakes:

1. After cupcakes have cooled for about 10 minutes, pierce tops of cupcakes with a sharp knife or skewer about 10 times. Pour about 1 tablespoon of caramel sauce on top of each cupcake, allowing it to absorb into the holes.
2. Let the cupcakes finish cooling completely and then frost with about 2 tablespoons of frosting on each. Lightly sprinkle your cupcakes with (coarse or flaked) sea salt.