01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners and set aside.
02 - In a medium bowl, sift together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt. Whisk gently to distribute evenly.
03 - In a large mixing bowl, beat the softened butter and granulated sugar with an electric mixer on medium-high speed until pale, light, and fluffy, about 3 minutes.
04 - Add the eggs one at a time, beating well after each addition until fully incorporated. Stir in the vanilla extract.
05 - Add the sifted flour mixture in three additions, alternating with the whole milk in two additions, beginning and ending with the flour mixture. Mix on low speed just until each addition is incorporated. Stir in the hot water by hand until the batter is smooth and uniform.
06 - Divide the batter evenly among the prepared liners, filling each about two-thirds full. Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean. Transfer to a wire rack and cool completely before filling.
07 - In a mixing bowl, beat the softened butter and powdered sugar together until light and creamy. Add the marshmallow creme and vanilla extract, then mix on medium speed until smooth and well combined.
08 - Transfer the marshmallow filling to a piping bag fitted with a small round tip. Insert the tip into the center of each cooled cupcake and squeeze gently to fill. Reserve a small amount of filling if desired for a decorative swirl on top.
09 - Heat the heavy cream in a small saucepan until steaming but not boiling. Pour the hot cream over the chopped semi-sweet chocolate in a heatproof bowl. Let stand for 2 minutes, then stir gently until the ganache is glossy and completely smooth. Allow to cool for 5 minutes until slightly thickened.
10 - Dip the top of each filled cupcake into the ganache, allowing any excess to drip off. Place on a wire rack and let the ganache set for about 10 minutes.
11 - Stir together the powdered sugar and milk to form a thick but pipeable icing. Transfer to a small piping bag or zip-top bag with a tiny corner snipped off. Pipe the classic squiggly swirl pattern across the top of each cupcake. Allow the icing to set before serving.