Preheat oven to 350°F. Line a 12-cup muffin tin with cupcake liners.

In a large bowl, whisk together melted butter and sugar until combined. Whisk in the eggs one at a time until just incorporated. Whisk in sour cream and vanilla extract.

Add the dry ingredients right on top of the wet ingredients. Using a wooden spoon or rubber spatula, mix until thoroughly combined. Fold in the mini chocolate chips.

Fill each tin 2/3 full with batter. I find it easiest to use a 1/4-cup cookie scoop. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out mostly clean (a few crumbs are okay). Allow the cupcakes to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.

Using a paring knife, cut out a 1-inch round hole in the center of each cupcake that measures about 1-inch deep. After you have all of the cupcakes cored, drizzle in 1 teaspoon of hot fudge, give or take.

Add heavy cream, sugar, cocoa, and salt to a saucepan; stir to combine. Bring the mixture to a boil over medium heat.

Remove from the heat and whisk in the chocolate, butter, and vanilla until smooth.

Transfer the fudge to an airtight container or mason jar. The fudge will stay fresh for up to 1 week in the refrigerator.

In the bowl of a stand mixer fitted with the paddle attachment, cream butter until light and fluffy (about 2 minutes).

Turn the mixer down to low and gradually add the powdered sugar to the bowl, stopping the mixer to scrape down the sides of the bowl every few minutes. Beat in the malted milk powder. Turn the speed up to medium and beat for 2 minutes.

Scrape down the sides of the bowl and add the vanilla extract. Give the mixture a few good turns. Turn the speed down to low and slowly drizzle in the heavy cream.

Once all of the heavy cream has been added, turn the speed up to medium and whip for 1 minute. Scrape down the sides of the bowl and give it one more good whip.

Transfer the buttercream to a piping bag attached with a round #14 tip. Or, you can skip the piping bag and add your buttercream to the cupcakes with a cake spatula or butter knife. If you're using a piping bag, pipe a swirl of frosting over the pool of frosting of each cupcake. Drizzle with extra hot fudge.