In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, cocoa powder, baking soda, and salt, to disperse all ingredients.

In another bowl, mix together melted butter, oil, buttermilk, sourdough discard, eggs, and vanilla extract. Mix just with a hand mixer (or very vigorously with a fork) until the batter becomes smooth and the sourdough discard is nicely incorporated. Pour it into the dry ingredients.

Divide the batter evenly between the prepared cake pans and bake for 30–35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Let the cakes cool in their pans for about 10 minutes before carefully turning them out onto a cooling rack. Allow them to cool completely before frosting.

While the cakes cool, melt the chocolate gently over boiling water or in short microwave intervals. Set it aside for a few minutes so it’s still smooth and pourable, but not extremely hot.

In a large bowl, mix the mascarpone and Nutella until you have a uniformly colored mixture. It’s ok if it’s quite stiff, especially if you used mascarpone straight from the fridge. Slowly pour in the melted chocolate while mixing continuously, until you have a smooth chocolate mixture. Again, it’s ok if it’s stiff, because we’re going to use it as frosting, so it shouldn’t really be runny.

In a separate bowl, whip the heavy cream until medium-stiff peaks form. Be careful not to overwhip it.

Using a spatula, gently fold the whipped cream into the chocolate mascarpone mixture until the frosting becomes light, fluffy, and evenly combined.

Once the cake layers are fully cooled, spread frosting between the layers and over the top and (optionally) the sides of the cake.