Yellow Sheet Cake with Chocolate Sour Cream Frosting

Preparation

1. Cake

   Step 1

   Preheat oven to 350°. Lightly coat a 18x13" rimmed baking sheet with nonstick spray. Line with parchment paper; coat parchment with nonstick spray. Whisk flour, baking powder, baking soda, and salt in a large bowl until no lumps remain. Combine buttermilk, oil, and vanilla in a medium bowl.

   Step 2

   Combine granulated sugar, brown sugar, and butter in another large bowl. Scrape in seeds from vanilla bean; save pod for another use. Using an electric mixer on medium speed, beat until light and fluffy, about 4 minutes. Add egg yolks and eggs one at a time, beating to blend after each addition. Continue to beat mixture, occasionally scraping down sides and bottom of bowl, until almost doubled in volume and very light, airy, and pale yellow, about 5 minutes. It’s very important that there are no lumps at this stage. The occasional scraping of the bowl, especially the bottom, will help prevent lumps.

   Step 3

   Reduce speed to low, add dry ingredients in 3 additions, alternating with buttermilk mixture in 2 additions, beginning and ending with dry ingredients. Scrape batter into prepared baking sheet and smooth top.

   Step 4

   Bake cake, rotating sheet halfway through, until light golden brown, the center springs back when gently pressed, and a tester inserted into the center comes out with a few moist crumbs attached, 20–25 minutes. Transfer baking sheet to a wire rack and let cake cool.

   Step 5

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

   Step 6

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

2. Frosting and Assembly

   Step 7

   Heat chocolate and corn syrup in a heatproof bowl set over a saucepan of simmering water (water should not touch bottom of bowl), stirring constantly, until chocolate is melted and mixture is smooth; remove from heat.

   Step 8

   Place sour cream, salt, and vanilla in another heatproof bowl and set over same saucepan of simmering water. Stir until mixture is warm but not hot, about 2 minutes. Add sour cream mixture to chocolate mixture and stir until smooth and glossy.

   Step 9

   Using an electric mixer on medium speed, beat powdered sugar and butter in a large bowl until light and fluffy, about 3 minutes. Scrape down sides of bowl, then slowly beat in chocolate mixture. Continue beating until no lumps remain, about 2 minutes. Frosting should be smooth and shiny. Cover and chill until slightly firmed up, about 30 minutes. Spread frosting over top of cake.

   Step 10

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