Chocolate Sheet Cake With Crème Fraîche Frosting

Preheat oven to 350˚. Butter an 18x13" rimmed baking sheet and line with parchment paper. Butter parchment and dust with flour, tapping out excess flour.

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

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, vegetable oil, ½ cup butter, and ½ cup 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 and beating until smooth after each addition. Scrape batter into prepared sheet and smooth top.

Bake cake until top is firm to the touch and a tester inserted into the center comes out clean, 25–35 minutes. Let cool in baking sheet.

Using a long serrated knife, cut off any uneven edges or bumps from cake to create a flat surface.

Do Ahead: Cake can be baked 1 day ahead. Store tightly wrapped at room temperature, or freeze up to 1 month.

Using an electric mixer on low speed, beat cream cheese and butter in a medium bowl until combined and smooth, about 3 minutes. Add crème fraîche and beat until smooth. Gradually add powdered sugar and beat until combined and smooth, then beat in vanilla and salt. Spread frosting over top of cake.

Do Ahead: Cake can be assembled 1 day ahead; chill. Bring to room temperature before serving.