Preheat your oven to 350°F (175°C) and prepare two 8-inch round cake pans by greasing them with butter and dusting with flour.

In a large mixing bowl, sift together the all-purpose flour, granulated sugar, cocoa powder, baking soda, baking powder, and kosher salt. Set aside.

In a separate bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract until smooth. Set aside.

Gradually combine the wet ingredients with the dry mixture using a stand mixer on low speed and scrape down the bowl as needed.

Carefully stir in the hot coffee until combined.

Pour the batter evenly into the prepared cake pans and bake for 35-40 minutes or until a toothpick comes out clean.

Let the cakes cool in the pans for about 30 minutes, then turn them out onto a wire rack to cool completely.

To make the frosting, melt the semisweet chocolate and let it cool slightly. Beat the softened butter until creamy, then add the cooled chocolate, egg yolk (if using), and vanilla extract.

Gradually mix in the sifted powdered sugar until the frosting is smooth. Add cream if too thick.

Once the cake layers are cool, frost one layer on a serving plate, add frosting, and place the second layer on top. Frost the top and sides.