For the cupcakes:
2½ cups all-purpose flour
1 tbsp. baking powder
½ tsp. salt
12 tbsp. unsalted butter, at room temperature
2½ cups sugar
1 cup plus 2 tbsp. buttermilk
1 1/3 cups mashed ripe banana
For the caramel:
8 tbsp. unsalted butter
½ cup heavy cream
1 cup packed light brown sugar
For the frosting:
3 large egg whites, at room temperature
¾ cup sugar
Pinch of salt
16 tbsp. unsalted butter, cut into 1 tbsp. pieces, at room temperature
To make the cupcakes, preheat the oven to 350° F. Line two cupcake pans with paper liners. In a medium bowl combine the flour, baking powder and salt; whisk together and set aside. In the bowl of a stand mixer fitted with the paddle attachment cream together the butter and sugar on medium-high speed until light and fluffy, about 2 minutes. Reduce the mixer speed to medium low and add in eggs one at a time, scraping down the sides of the bowl as needed. Mix in the mashed banana until well combined. With the mixer on low speed alternately add the dry ingredients and buttermilk, beginning and ending with the dry ingredients and mixing only until just incorporated.
Divide the batter between the prepared cupcake liners, filling each about 2/3 to ¾ full. Bake until a toothpick inserted in the center comes out clean, about 20-22 minutes. Allow to cool in the pan 5-10 minutes, then transfer the cupcakes to a wire rack to cool completely.
To make the caramel sauce, combine the butter, heavy cream and brown sugar in a medium heavy-bottomed saucepan over medium-high heat. Bring the mixture to a boil, stirring constantly. Once it reaches a boil, lower the heat to medium and continue to boil until the mixture is thickened, about 8 minutes. Remove from heat and let cool.
To make the frosting, combine the egg whites and sugar in a large heatproof bowl set over a pot of simmering water. Heat the mixture, whisking very frequently, until the sugar is completely dissolved and reaches 160° F on an instant read thermometer. Transfer the mixture to the clean, dry bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed until the mixture holds stiff peaks. Reduce the speed to medium low, add in the salt and begin adding the butter one piece at a time, beating well after each addition. Stir in ½ cup of the cooled caramel sauce until well blended.
Frost the cooled cupcakes with the caramel buttercream, and drizzle with the remaining caramel sauce. (If the caramel has cooled too much to be easily drizzled, heat for 10 seconds in the microwave and stir well to thin it a bit.)