For the cupcakes:

1. Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.
2. In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
3. Add the eggs, vanilla extract, milk, and vegetable oil to the dry ingredients. Mix until smooth and combined.
4. Gradually add the boiling water to the batter, stirring gently. The batter will be thin, but that’s normal.
5. Divide the batter evenly between the cupcake liners, filling each about two-thirds full.
6. Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
7. Allow the cupcakes to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.

For the salted caramel frosting:

1. In a medium saucepan over medium heat, melt the butter and brown sugar, stirring constantly until the mixture comes to a boil.
2. Let the caramel simmer for 2-3 minutes, then remove from the heat and stir in the heavy cream, vanilla extract, and sea salt.
3. Let the caramel mixture cool to room temperature.
4. Once cooled, gradually beat in the powdered sugar until smooth and fluffy.

Assembling the cupcakes:

1. Once the cupcakes have cooled completely, pipe or spread the salted caramel frosting onto each cupcake.
2. Garnish with a sprinkle of sea salt flakes and drizzle with additional caramel sauce if desired.