Simple Two-Layer Chocolate Birthday Cake with Milk Chocolate Ganache

Preheat oven to 350˚. Butter two 9"-diameter cake pans and line bottoms with parchment paper rounds. Butter parchment and dust with flour, tapping out excess.

Heat chocolate, coffee, and ⅔ cup cocoa powder in a medium heatproof bowl set over a medium saucepan of barely simmering water (water should not touch bottom of bowl), stirring until chocolate melts and mixture is smooth. Let cool, then whisk eggs, buttermilk, and vanilla into chocolate mixture.

Using an electric mixer on low speed, mix salt, baking powder, baking soda, and 2 cups flour in a large bowl just to combine. Add brown sugar, oil, ½ cup butter, and ½ cup reserved chocolate mixture and beat on medium speed until flour is evenly distributed and mixture is smooth, about 2 minutes. Add remaining chocolate mixture in 2 additions, scraping down sides and bottom of bowl as needed and beating until smooth after each addition. Scrape batter into prepared pan and smooth top.

Bake cake until top is firm to the touch and a tester inserted into the center comes out clean, 30–40 minutes. Transfer cake pan to a wire rack and let cake cool in pan 10 minutes before turning out onto a wire rack. Let cool completely.

Place chocolate in a medium bowl. Bring cream and salt to a simmer in a small saucepan over medium-low heat. Pour over chocolate; let sit 10 minutes, then whisk gently until chocolate is melted and mixture is smooth and glossy. Let ganache cool, 30–45 minutes.

Using an electric mixer on medium speed, beat butter into ganache until mixture is light and fluffy and has turned matte, about 3 minutes.

Place a cake layer on a platter. Using a rubber spatula, spread half of ganache over top of cake, spreading to the edges; top with remaining cake layer. Spread remaining ganache over top of cake, leaving sides bare. Chill 1–8 hours before serving.