01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, beat the softened butter and granulated sugar with an electric mixer until light and fluffy, about 3 minutes. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
04 - Add the flour mixture in three parts, alternating with the milk, beginning and ending with the flour mixture. Mix just until combined. Stir in the hot water until the batter is smooth and uniform.
05 - Divide batter evenly among the 12 liners, filling each about two-thirds full. Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean. Let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
06 - Beat the softened butter and powdered sugar together until light and fluffy. Add marshmallow creme and vanilla extract, then mix until smooth and well combined.
07 - Transfer 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 the swirl decoration.
08 - Heat heavy cream in a small saucepan until steaming but not boiling. Pour over chopped semi-sweet chocolate in a heatproof bowl. Let sit for 2 minutes, then stir until glossy and smooth. Allow to cool for 5 minutes until slightly thickened.
09 - Dip the top of each filled cupcake into the ganache, allowing excess to drip off. Place on a wire rack and let set for 10 minutes until the ganache firms up.
10 - Stir together powdered sugar and milk to form a thick but pipeable icing. Transfer to a small piping bag and pipe signature squiggly lines across the top of each cupcake. Allow icing to set before serving.