Heat oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners.
Whisk dry ingredients: flour, sugar, cocoa, baking soda, baking powder, and salt.
Whisk wet ingredients: buttermilk, oil, eggs, and vanilla until smooth.
Combine wet with dry. Stir just until no dry streaks remain.
Pour in hot coffee and whisk gently. The batter will look thin—that’s correct.
Fill liners about 2/3 full. Bake 16–20 minutes, until tops spring back and a toothpick comes out with a few moist crumbs.
Cool in the pan 5 minutes, then transfer to a rack to cool completely.
(Optional stabilization) Sprinkle gelatin over 1 tablespoon cold water. Let bloom 5 minutes, then microwave 5–10 seconds to melt. Cool until just warm.
Beat cold heavy cream with powdered sugar and vanilla to soft peaks.
Drizzle in the melted gelatin while mixing to medium peaks.
Fold in marshmallow creme until fluffy and smooth. Chill 15 minutes to thicken for piping.
Use a cupcake corer, apple corer, or a small knife to remove a plug from the center of each cooled cupcake. Do not cut to the bottom; leave a base.
Pipe or spoon in the marshmallow filling until level with the top.
Trim the removed cake plugs and gently cap the filling. You only need a thin lid.
Heat heavy cream just to a simmer. Pour over chopped semi-sweet chocolate and a pinch of salt. Let sit 2 minutes.
Whisk from the center outward until smooth and glossy. Cool 5–10 minutes until slightly thick but still pourable.
Dip each filled cupcake top-first into the ganache, letting excess drip off. Set on a rack to set 15–20 minutes.
Beat together powdered sugar, butter, vanilla, and a splash of milk to make a thick white icing that holds its shape.
Transfer to a piping bag fitted with a small round tip and pipe the classic looped squiggle across each cupcake.
