Line a muffin pan with 12 cupcake liners. Preheat oven to 350 degrees F.

In a medium bowl, whisk together flour, baking powder, and salt. Set aside.

In the bowl of your stand mixer, cream together butter, brown sugar, and granulated sugar. Add eggs, one at a time, beating well after each addition, mix in vanilla.

Add flour mixture and milk alternatively. Start and end with the flour. Be careful not to over mix the batter, but make sure no lumps remain. Divide batter evenly between the 12 cupcake cavities. Place in the oven and bake for 20-25 minutes, or until a toothpick inserted in the middle comes out clean. Let stand for 5 minutes, then transfer cupcakes to wire racks to cool completely.

While the cupcakes are cooling, prepare the salted caramel sauce.

In a medium saucepan, heat sugar over medium-high heat, stirring constantly (using either a wire whisk or a wooden spoon). Continue to stir until sugar is completely dissolved and no lumps remain. Melted sugar should be a golden brown color.

Add butter to the sugar and place a thermometer into the hot mixture. Butter should be completely melted and mixture should reach 350 degrees F. Immediately remove from the stove and sprinkle in sea salt. Give it a good stir and slowly whisk in heavy cream. Continue stirring until heavy cream is fully incorporated and smooth. Let sauce cool as well before piping into the cooled cupcakes. Fill a piping or ziptop bag fitted with a round tip with about half of the salted caramel sauce. Fill the center of each cupcake with caramel. Cupcakes are filled when you feel pressure while piping and once you remove the round tip, a small amount of caramel starts to ooze out. Set cupcakes aside.

Next, prepare the buttercream. Remove butter from the fridge, cut into cubes and leave on the counter nearby.

Add egg whites and granulated sugar to the bowl of your stand mixer, place over a pot of simmering water (not boiling), whisking gently and frequently until the sugar has completely dissolved and the egg whites are hot to the touch.

With whisk attachment of your mixer, whip until the egg whites are glossy, and the bottom of the bowl feels neutral when touched (aka no longer warm). This took me about 14 minutes. Don't add butter before bowl has had a chance to cool or butter will melt and you won't have the desired outcome.

Switch over to the paddle attachment and, with mixer on low speed, add butter cubes, one at a time, until incorporated, and mix until it has reached a smooth texture (if mixture curdles, keep mixing and it will come back to smooth). If mixture appears to be too runny, refrigerate for about 15 minutes and continue mixing with paddle attachment until it comes together. Add vanilla and salt, continuing to beat on low speed until well combined.

Fill a large piping bag fitted with a medium star tip and pipe buttercream onto each cupcake. Drizzle with remaining salted caramel sauce and garnish with soft caramels, if desired. Store cupcakes in the fridge until ready to serve.