1) For the ganache: Place the chopped chocolate in a medium-sized bowl. Heat the cream until just boiling and pour over the chocolate. Slowly stir until completely melted and lump-free. Set the bowl in the fridge to chill while you make the cake.
2) Preheat the oven to 350°F and prepare your chosen pan(s) by spritzing well with cooking spray.
3) For the cake: Place the oil, butter, sugar, and eggs in the bowl of your mixer. Beat on medium speed for 2 minutes. Stop the mixer and scrape down the bowl.
4) Add the melted milk chocolate, salt, flour, Cake Enhancer, baking soda, vanilla, dried whole milk, cocoa powder and 1/2 cup of the buttermilk. Beat on medium speed for 1 minute. Stop again and scrape the bowl.
5) Add the remaining buttermilk and beat for 30 seconds.
6) Pour the batter into the pan(s), and bake: for 8" rounds, 18 to 24 minutes; for 9" rounds, 20 to 28 minutes; for cupcakes, 15 to 18 minutes. For a 13" x 9" x 2" pan, bake 25 to 35 minutes. The cake is done when a toothpick inserted into the center comes out clean, with just a few moist crumbs.
7) Rest the cake in the pan for 10 minutes, then turn out onto a rack to cool completely.
8) To prepare the ganache: The chilled ganache should now be the consistency of chocolate pudding. Place it in the bowl of your mixer and whip with the whisk attachment until thick, light, and fluffy. Be sure to scrape the bottom of the bowl to fully incorporate all of the chocolate.
9) To assemble the cake: Using a long serrated knife, divide each cake round into two layers. Place one layer on your serving plate and spread with a 1/2"-thick layer of the whipped ganache. Add another layer of cake.
10) If you're using Marshmallow Fluff for filling, pipe a ring of ganache around the top of the second layer as a border, then fill it with 3/4 to 1 cup Fluff. The border of ganache will keep the marshmallow from oozing out the sides of the cake.
11) Add another layer of cake, another layer of ganache, and the final layer of cake. Ice the rest of the cake with the remaining ganache.
12) Serve at room temperature; refrigerate any leftovers, well-wrapped, for up to 5 days.
Yield: one 8" or 9" layer cake; or one 9" x 13" sheet cake; or 24 cupcakes.