Preheat the oven to 350°F and grease two 9-inch round cake pans. In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt. Add buttermilk, oil, eggs, and vanilla, then mix until smooth. Stir in hot water until fully combined. Divide the batter between the pans and bake for 28-30 minutes. Let cakes cool completely.
In a mixing bowl, beat butter until light and fluffy. Gradually add powdered sugar, then mix in marshmallow creme and vanilla. Continue whipping until smooth and airy.
Heat heavy cream in a saucepan until steaming but not boiling. Remove from heat and pour over chopped chocolate. Let sit for 5 minutes, then stir until smooth and glossy. Allow the ganache to cool slightly.
Place one cake layer on a serving plate. Spread the cream filling evenly over the top. Place the second cake layer on top and gently press down.
Pour the ganache over the top of the cake, letting it drip down the sides. Smooth with a spatula if needed. Refrigerate for at least 30 minutes before slicing. Enjoy this chocolatey treat with a side of Peanut Butter Cup Brownies!