Make the Cake
Preheat your oven to 350°F and grease two 9-inch round cake pans. Line the bottoms with parchment paper.
In a large bowl, whisk together the flour, cocoa powder, granulated sugar, brown sugar, baking soda, baking powder, and salt.
In a separate bowl, mix the buttermilk, vegetable oil, eggs, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Stir in the hot water until the batter is smooth and thin.
Divide the batter evenly between the prepared pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Let the cakes cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.
Prepare the Filling
In a mixing bowl, beat the softened butter until light and fluffy.
Add the marshmallow creme, powdered sugar, and vanilla extract, beating until smooth and fluffy. Set aside.
Make the Ganache
Heat the heavy cream in a small saucepan over medium heat until it begins to simmer (do not boil).
Remove from heat and pour over the chocolate chips in a bowl. Let sit for 2 minutes, then stir until smooth. Stir in the butter for a glossy finish. Let the ganache cool slightly until it thickens to a pourable consistency.
Assemble the Cake
Place one cake layer on a serving plate or cake stand. Spread the cream filling evenly over the top, leaving a slight border to prevent it from oozing out.
Place the second cake layer on top. Gently press down to secure.
Pour the ganache over the cake, letting it drip down the sides. Smooth the top with an offset spatula if needed.
Chill and Serve
Refrigerate the cake for at least 1 hour to set the ganache.
Slice and serve chilled or at room temperature.