Preheat the oven to 350°F.
Line a muffin pan with paper cupcake liners.
In a two-cup glass measuring cup, whisk together the Dutch-process cocoa powder and hot water until smooth. Set aside.
In a medium bowl, whisk together the all-purpose flour, baking soda, baking powder, and coarse salt. Set aside.
In a medium saucepan over medium heat, combine the unsalted butter and granulated sugar.
Stir occasionally until the butter is melted and the mixture is fully combined.
Remove the saucepan from the heat.
Transfer the butter and sugar mixture to the bowl of a stand mixer fitted with the paddle attachment.
Beat on medium-low speed for 4 to 5 minutes, or until the mixture has cooled slightly.
Add the eggs and egg yolk one at a time, beating well after each addition.
Scrape down the sides of the bowl as needed.
Add the vanilla extract and prepared cocoa mixture.
Mix until fully incorporated.
With the mixer on low speed, add the dry ingredients in two additions, alternating with the sour cream.
Mix only until the batter is combined. Do not overmix.
Divide the batter evenly among the prepared cupcake liners, filling each liner approximately three-quarters full.
Bake for 18 to 20 minutes, rotating the pan halfway through baking.
The cupcakes are done when a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the cupcakes to cool in the pan for 5 to 10 minutes.
Transfer the cupcakes to a wire rack and allow them to cool completely.
To prepare the caramel sauce, melt the butter in a heavy-bottomed saucepan over medium heat.
Stir in the salt and packed dark brown sugar.
Bring the mixture to a boil, stirring constantly.
Reduce the heat to low and cook for 2 minutes, or until the sugar is fully dissolved.
Stir in the milk.
Return the mixture to a boil, stirring constantly.
Remove the saucepan from the heat.
Stir in the vanilla extract.
Allow the caramel sauce to cool until lukewarm, about 30 to 40 minutes, stirring occasionally.
Once the cupcakes are fully cooled, use a paring knife or apple corer to remove a small circle from the center of each cupcake.
Remove and discard the cupcake cores.
In a medium bowl, combine the chopped 12 fun-size Snickers bars with 1/4 cup of the cooled caramel sauce.
Stir until the chopped candy bars are evenly coated.
Spoon the Snickers filling into the center of each cupcake.
Reserve 3 tablespoons of the caramel sauce for drizzling over the finished cupcakes.
Transfer the remaining caramel sauce to the bowl of a stand mixer fitted with the paddle attachment.
Add 3 1/2 cups powdered sugar.
Beat on medium-high speed for 3 to 4 minutes, or until the frosting is light and fluffy.
If the frosting is too thin, add additional powdered sugar until the desired piping consistency is reached.
If the frosting is too thick, add a small amount of milk until smooth and pipeable.
Transfer the caramel frosting to a pastry bag fitted with a decorative tip.
Pipe the frosting onto each cupcake, beginning at the outer edge and working inward.
Drizzle the frosted cupcakes with the reserved caramel sauce.
Garnish each cupcake with chopped Snickers bars.
Serve immediately or refrigerate until ready to serve.