Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add eggs, milk, vegetable oil, and vanilla extract. Beat on medium speed until well combined.
Stir in boiling water until the batter is smooth. The batter will be thin.
Divide the batter evenly between the prepared pans. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
Cool cakes in the pans for 10 minutes, then transfer to a wire rack to cool completely.
For the filling, beat butter, powdered sugar, marshmallow creme, and vanilla extract until light and fluffy.
Spread the filling evenly over one cooled cake layer. Place the second layer on top.
For the chocolate ganache, heat heavy cream until simmering. Pour over chopped chocolate and let sit for 5 minutes, then stir until smooth.
Pour ganache over the cake, spreading evenly. Let set before serving.