Preheat the oven to 350°F (175°C) and line standard muffin pans with paper liners. In a small bowl or measuring cup, whisk together the Dutch-process cocoa powder and hot water until smooth; set aside to bloom.

In a medium bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt. Set aside.

In a medium saucepan over medium heat, melt the butter with the granulated sugar, stirring occasionally until the mixture is smooth. Transfer to the bowl of a stand mixer fitted with the paddle attachment and beat on medium-low speed for 4 to 5 minutes until slightly cooled.

Add the eggs and egg yolk one at a time, mixing until fully incorporated and scraping down the sides of the bowl as necessary. Mix in the vanilla extract and the cocoa mixture until fully blended.

With the mixer on low speed, add the dry ingredients in two batches, alternating with the sour cream. Mix only until combined—do not overmix.

Divide the batter evenly among the prepared cupcake liners, filling each about three-quarters full. Bake for 18 to 20 minutes, rotating the pans halfway through, until a toothpick inserted into the center comes out clean. Allow cupcakes to cool in the pans for 5 to 10 minutes before transferring to a wire rack to cool completely.

In a heavy-bottomed saucepan, melt the butter over medium heat. Stir in the salt and dark brown sugar and bring the mixture to a gentle boil, stirring constantly. Reduce heat and simmer for 2 minutes, ensuring the sugar dissolves fully. Add the milk and return to a boil. Remove from heat and stir in the vanilla extract. Allow the sauce to cool to lukewarm temperature (about 30 to 40 minutes), stirring occasionally.

Once cupcakes have cooled, use a paring knife or apple corer to remove a small circle from the center of each cupcake. In a bowl, combine the chopped Snickers bars with ¼ cup of the cooled caramel sauce and stir until well coated. Fill each cupcake cavity with a spoonful of this Snickers mixture. Reserve 3 tablespoons of caramel sauce for final drizzling.

Transfer the remaining caramel sauce to the bowl of a stand mixer fitted with the paddle attachment. Add the powdered sugar and beat on medium-high speed for 3 to 4 minutes until light and fluffy. Adjust the consistency by adding additional milk or powdered sugar as needed.

Transfer the frosting to a piping bag fitted with a decorative tip. Pipe a spiral of frosting onto each filled cupcake, beginning at the outer edge and working inward. Drizzle with the reserved caramel sauce and garnish with chopped Snickers bars.