Preheat your oven to 350°F. It’s important to ensure that your oven is hot enough before you start baking, as this helps the cupcakes rise properly. While the oven heats, line standard muffin cups with cupcake paper liners or grease them with butter and dust with flour. This prevents sticking and ensures easy removal.
In a medium bowl, place ¾ cup natural cocoa powder and slowly whisk in ⅔ cup boiling water until fully incorporated. This step is crucial as it helps to bloom the cocoa, enhancing the chocolate flavor. Let the mixture cool slightly.
Whisk in 1 cup buttermilk to the cocoa mixture. This will add moisture and tanginess to the cupcakes, balancing the sweetness.
In a separate large bowl, whisk together 1¾ cups all-purpose flour, 1½ teaspoons baking soda, and ½ teaspoon salt. This dry mixture ensures an even distribution of the leavening agent, resulting in a perfectly risen cupcake.
In the bowl of a mixer fitted with a paddle attachment, beat ¾ cup unsalted butter and both sugars (1 cup sugar and ½ cup brown sugar) on medium speed until combined and fluffy. This process is known as creaming, and it incorporates air into the butter, resulting in a lighter cupcake.
One at a time, add 3 large eggs to the mixture, beating well after each addition. This helps to emulsify the mixture and ensures that everything is combined smoothly. Add 2 teaspoons vanilla extract with the last egg.
Reduce the speed to low and add the flour mixture in three additions, alternating with the cocoa-buttermilk mixture in two additions, starting and ending with the dry ingredients. This method prevents overmixing and helps to keep the cupcakes tender.
Divide the batter among the prepared muffin cups, filling them about three-quarters full. This will allow room for the cupcakes to rise without overflowing.
Bake the cupcakes for about 20 minutes, until they are puffed, slightly springy to the touch, and a toothpick inserted into the center comes out clean. This ensures that your cupcakes are perfectly baked.
Let the cupcakes cool slightly in the pan, then remove them and transfer to wire racks to cool completely. Cooling is crucial as it prevents the frosting from melting when applied.
Meanwhile, prepare the frosting. In a large bowl, place 6 tablespoons unsalted butter and 1½ cups confectioners' sugar. Beat on medium speed until lightened and fluffy.
Add 1 cup marshmallow cream, 1 teaspoon pure vanilla extract, and 2 tablespoons heavy cream. Beat until the mixture is light and fluffy. Set aside.
Using a small sharp knife, create a hole in the top of each cupcake about 1½ inches round and 1 inch deep. This will allow you to fill them with the delicious marshmallow filling.
Fill each cavity with the marshmallow filling you just prepared.
In another large bowl, beat together 8 oz cream cheese, ½ cup unsalted butter, and 2 teaspoons vanilla extract until light and fluffy. This will be the base for your frosting.
Beat in ½ cup unsweetened cocoa powder until combined, then gradually add 3 to 4 cups confectioners' sugar until a frosting consistency is reached. Adjust as necessary to achieve the desired creaminess.
Use an offset spatula or a pastry bag with a tip to frost the cupcakes generously. If you like, sprinkle with gold cupcake gems for an extra touch of flair!