Chocolate Birthday Sheet Cake

Preheat oven to 350˚. Butter and flour a 13x9" baking dish, tapping out excess flour.

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 15 minutes before turning out onto a wire rack. Let cool completely.

Just before serving, whisk cream in a small bowl to soft peaks. Add granulated sugar to taste and beat to medium peaks. Top cake with whipped cream and dust with a little cocoa powder.

Do Ahead: Cake (without whipped cream) can be baked 3 days ahead. Store tightly wrapped at room temperature.