• Mix the flour, cocoa powder, salt, baking powder, and baking soda in a large bowl. Set it aside.
• Whisk together the sourdough starter, eggs, vanilla, oil, milk, and sugars in a separate bowl.
• Pour the wet mixture into the flour mixture and fold gently for a minute before pouring over the hot coffee.
• Mix briskly together but don't over-mix.
• Fermenting overnight (optional) - Cover the bowl tightly and refrigerate for up to 12 hours.
• Heat the oven to 180°C (350°F). Grease and line two 8-inch round cake pans.
• Pour into the prepared cake pans and bake for 25-30 minutes until a skewer inserted into the cakes comes out clean.

Chocolate Buttercream

• In a stand mixer bowl fitted with a paddle attachment, beat the room-temperature butter until light and creamy. 
• Add the powdered sugar, cocoa powder, milk, vanilla, and salt to the butter. 
• Whisk on low speed to incorporate them, scraping the sides of the bowl as needed. Once incorporated, switch to medium-high speed and beat the mixture for another 1-2 minutes until it's light and fluffy.
• Spread 1/3 of the buttercream on one cake round so it reaches the edges. Add the second cake round on top. Spread with the remaining buttercream.