For the cupcakes:
½ cup Dutch-process cocoa powder
½ cup hot water
2 cups all-purpose flour
¾ tsp. baking soda
¾ tsp. baking powder
¾ tsp. coarse salt
16 tbsp. unsalted butter
1½ cups sugar
2 large eggs plus 1 egg yolk
1¾ tsp. vanilla extract
2/3 cup sour cream, at room temperature
For the caramel sauce:
8 tbsp. unsalted butter, cut into pieces
1 cup sugar
1 cup heavy cream
¼ tsp. vanilla extract
Pinch of salt
For the filling:
24 fun-size Snickers bars, chopped
For the frosting:
16 tbsp. unsalted butter, at room temperature
1 lb. confectioners’ sugar
1½ tsp. vanilla extract
1/3 cup caramel sauce
Pinch of coarse salt
2 tbsp. heavy cream
8 fun-size Snickers bars, chopped
To make the cupcakes, preheat the oven to 350˚ F. Line standard cupcake pans with paper liners. In a small bowl, whisk together the cocoa powder and hot water until smooth. In a medium bowl, whisk together the flour, baking soda, baking powder, and salt; set aside.
In a medium saucepan, combine the butter and the sugar over medium heat. Heat, stirring occasionally to combine, until the butter is melted. Remove the mixture from the heat and transfer to the bowl of an electric mixer fitted with a paddle attachment. Beat on medium-low speed, 4-5 minutes, until the mixture is cooled. Mix in the eggs and egg yolk, one at a time, scraping down the sides of the bowl as needed and beating well after each addition. Mix in the vanilla and then the cocoa mixture and beat until incorporated. With the mixer on low speed add in the dry ingredients in two batches, alternating with the sour cream, beating just until combined.
Divide the batter between the prepared cupcake liners, filling them about ¾ of the way full. Bake 18-20 minutes or until a toothpick inserted in the center comes out clean, rotating the pans halfway through baking. Allow the cupcakes to cool in the pan 5-10 minutes, then transfer to a wire rack to cool completely.
To make the caramel sauce, melt the butter in a heavy-bottomed saucepan over medium heat. Add the sugar and cook, stirring occasionally, until the sugar begins to foam a bit. It will look and smell like it’s on the verge of burning. Remove from the heat and add the heavy cream. Stir until the sauce is smooth (you may need to return it to the heat to smooth it out), then mix in the vanilla and salt. Let cool. (This can be stored in the refrigerator for up to 2 weeks.)
To fill the cupcakes, cut a cone out of the center of each cupcake with a paring knife.
To make the filling, combine the chopped Snickers bars in a bowl with 1/3-½ cup of the caramel sauce, and mix to coat.
Drop a spoonful of the filling mixture into each cupcake.
To make the frosting, add the butter to the bowl of an electric mixer. Beat on medium-high speed 1 minute until smooth. Blend in the confectioners’ sugar until smooth, 1-2 minutes. Mix in the vanilla, caramel sauce and salt until incorporated. (Note: the caramel sauce should be just warm enough that it is workable, but not warm enough to melt the butter in the frosting.) Add the heavy cream and whip on high speed until light and fluffy, about 3-4 minutes.
Transfer the frosting to a pastry bag fitted with a decorative tip. Drizzle the frosted cupcakes with additional caramel sauce and garnish with chopped Snickers bars.