For the Caramel Sauce:

1. In a medium heavy-bottomed saucepan, heat sugar over medium heat, stirring occasionally with a heat-resistant spatula until it melts and turns amber (about 8-10 minutes).
2. Once sugar is completely melted and amber colored, carefully add butter (mixture will bubble). Stir until butter is melted.
3. Slowly pour in heavy cream while stirring (mixture will bubble vigorously). Continue stirring until smooth.
4. Remove from heat, stir in sea salt, and let cool completely. Sauce will thicken as it cools.

For the Cupcakes:

1. Preheat oven to 350°F (175°C). Line a standard muffin tin with paper liners.
2. In a large bowl, cream together butter and sugar until light and fluffy, about 3-4 minutes.
3. Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
4. In a separate bowl, whisk together flour, baking powder, and salt.
5. In a small bowl or measuring cup, combine milk and sour cream.
6. Gradually add dry ingredients to butter mixture, alternating with milk mixture, beginning and ending with dry ingredients. Mix just until combined.
7. Fill cupcake liners two-thirds full with batter.
8. Drizzle about 1 teaspoon of caramel sauce over each cupcake and use a toothpick to swirl it into the batter.
9. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
10. Cool in pan for 5 minutes, then transfer to a wire rack to cool completely.

For the Buttercream Frosting:

1. Beat softened butter at medium speed until creamy, about 3 minutes.
2. Gradually add powdered sugar, caramel sauce, vanilla, and salt, beating at low speed until incorporated.
3. Add heavy cream 1 tablespoon at a time until desired consistency is reached.
4. Beat at medium-high speed until light and fluffy, about 2 minutes.

Assembly:

1. Once cupcakes are completely cool, pipe or spread frosting onto each cupcake.
2. Drizzle with additional caramel sauce.
3. Sprinkle with a small amount of flaky sea salt.
4. If desired, garnish with caramel pieces or chopped nuts.