Preheat the oven to 350°F. Grease two 9-inch cake pans, line with parchment, grease again, and dust with cocoa powder.
Whisk flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl.
Add eggs, buttermilk, oil, and vanilla. Mix until smooth.
Slowly pour in hot coffee (or hot water) while mixing on low. Mix just until combined.
Divide batter between pans. Bake 28–35 minutes, until a toothpick comes out with a few moist crumbs. Cool 15 minutes in pans, then cool completely on a rack.
Make the filling. Beat butter until smooth. Add marshmallow creme and beat until fluffy. Add powdered sugar, vanilla, and salt and beat again. Add heavy cream and beat 2–3 minutes until light.
Assemble. Level cakes if needed. Spread filling over the first layer, leaving a ½-inch border. Add the second layer. Chill 20–30 minutes.
Make ganache. Place chocolate in a bowl. Heat cream until steaming. Pour over chocolate, rest 2 minutes, then whisk smooth. Stir in corn syrup if using. Cool 10–15 minutes until slightly thickened.
Pour ganache over the chilled cake and spread gently. Refrigerate 30 minutes to set before slicing.