Place the butter, starter, flour, sugar, cocoa powder and buttermilk in a large bowl and stir until combined. Cover tightly with plastic wrap and place in fridge overnight to ferment.

The following day, remove the sourdough mixture from the fridge and let it come to room temperature, about 5 hours.

Preheat the oven to 350ºF and adjust the rack to the middle position.

Combine celtic sea salt, vanilla, baking soda, eggs, sour cream and instant coffee in a medium bowl. Add to the sourdough mixture and stir until smooth and combined. Pour the batter into a buttered 11×7” ceramic or glass baking dish. Bake for 40-45 minutes, until cake tester comes out clean. Cool completely.

Top with vanilla buttercream frosting, your favorite frosting recipe, or serve by itself.

For the buttercream: In the bowl of a standing mixer, combine eggs, maple syrup or honey, vanilla and pinch of salt. Set the bowl over a pot of simmering water. Whisk constantly until the mixture reaches 160 degrees, about 5 minutes. Pour into the bowl of a standing mixer. Beat egg mixture on medium-high with whisk attachment until light and billowy, about 8 minutes. Reduce the speed to medium and add butter one tablespoon at a time. When all of the butter is added, turn off the mixer and scrape down the sides of the bowl (at this point the buttercream might look curdled, but don’t worry, it will turn creamy). Whisk the entire mixture for 1 minute on high until light and fluffy. Frost the cake and enjoy!