Preheat your oven to 350°F (175°C) and grease and flour two 9-inch round cake pans.

In a mixing bowl, whisk together the all-purpose flour, black cocoa powder, baking soda, baking powder, and salt.

In another bowl, beat the unsalted butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes. Gradually incorporate the eggs and vanilla extract.

Alternately add the dry mixture to the butter mixture, stirring well after each addition. Then pour in the buttermilk and mix until combined without overmixing.

Pour the batter into the prepared pans and bake for 30-35 minutes until a toothpick comes out clean. Cool in the pans for 10 minutes before transferring to wire racks.

Heat the heavy cream in a saucepan, then pour it over the chopped semisweet chocolate. Stir until smooth, then mix in the softened butter and vanilla extract. Allow to cool until thickened.

Once cooled, place one cake layer on a serving plate, spread ganache on top, add the second layer, and coat with remaining ganache.

Chill the assembled cake in the fridge for about 30 minutes before serving at room temperature.