Preheat the oven to 350 degrees. Prepare a muffin pan by lining it with paper cupcake liners.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar until light and fluffy on medium speed. Add the eggs, and mix until well blended.
Add the buttermilk, cocoa powder, and whole milk to the batter. Mix until well combined.
Place a sieve over a large bowl. Pour the bread flour, baking soda, and salt into the sieve and sift. Add the sifted bread flour, baking soda, and salt to the batter in the standing mixer bowl. Mix until well combined. The mixture will be on the thicker side.
Using a spoon, scoop batter into each muffin cup, filling the cups halfway. Bake for 17-20 minutes or until a toothpick inserted in the center comes out clean. Allow the cupcakes to cool in their tins for at least 10 minutes. Then transfer them onto wire cooling racks and allow them to cool completely.
Once the cupcakes are thoroughly cooled, using a serrated knife - trim the cupcakes so they are completely flat. Next, using the back of a wooden spoon's handle - make a hole in the top center of each cupcake. Carefully rotate the handle in each hole to make the cavity slightly wider.
Cream the butter on medium speed in the bowl of a stand mixer, fitted with a whisk attachment. Add the powdered sugar and marshmallow fluff - beat on medium speed. Mix until smooth and creamy.
Place the marshmallow cream filling into a disposable piping-bag, fitted with a Wilton 8 piping tip. Pipe just enough marshmallow cream filling into each center cavity of the cupcakes. Remove any excess with a butter knife.
In a small saucepan, over medium heat, pour heavy cream and heat to a simmer. Meanwhile, add chocolate morsels to a medium bowl.
Once the cream begins to simmer, pour over the chocolate. Do not stir. Cover with plastic wrap and set aside for 5 minutes. After 5 minutes, whisk until smooth.
Using a spoon, place a dollop of chocolate ganache on each cupcake. Use the back of the spoon to smooth out the chocolate.
Cream the butter on medium speed in the bowl of a stand mixer fitted with a whisk attachment. Add the powdered sugar and salt- beat on low speed. Mix until the sugar and butter are combined. Add the milk to adjust the thickness of the frosting.
Place the vanilla buttercream into a disposable piping bag fitted with a Wilton 5 piping tip. Pipe the curlicue pattern on top of each cupcake.
Store your cupcakes in an airtight container in the refrigerator. Enjoy!
We've used a bread flour in our recipe for a denser crumb to ensure the stability of the cupcake to hold the filling and heavier ganache topping. But, you may choose to use all-purpose flour if you please.