Yellow Layer Cake with Chocolate-Sour Cream Frosting

Arrange racks in upper and lowerthirds of oven; preheat to 350°F. Coat cakepans with nonstick spray. Line bottom ofpans with parchment-paper rounds; coatpaper.

Whisk flour, baking powder, bakingsoda, and salt in a large bowl until no lumpsremain.

Combine buttermilk, oil, and vanillain a medium bowl.

Combine sugar, butter, and brown sugarin another large bowl. Scrape in seedsfrom vanilla bean. Using an electric mixer,beat butter mixture until light and fluffy,3–4 minutes.

Add yolks and eggs one at atime, beating to blend between additionsand occasionally scraping down sides andbottom of bowl.

Continue to beat mixture, occasionallyscraping down sides and bottom of bowl,until almost doubled in volume and verylight, airy, and pale yellow, 5–6 minutes. It'svery important that no lumps remain at thisstage. (Occasional scraping of the bowl,especially the bottom, helps prevent lumps.)

With mixer on low, add dry ingredientsin 3 additions, alternating with buttermilkmixture in 2 additions, beginning andending with dry ingredients.

Divide batter evenly among preparedpans, about 3 1/2 cups per pan. Smooth tops.

Bake, rotating cakes from left to rightand top to bottom halfway through, untilcakes are light golden brown, the centersspring back when gently pressed, and atester inserted into the centers comes outwith a few moist crumbs attached, 35–40minutes.

Transfer cakes to wire racks; let cool inpans for at least 30 minutes. Invert cakesonto racks, peel off parchment, and let coolcompletely.

Using a long serrated knife, remove topdome and any bumps from each cake tocreate a flat surface. Slice each cake inhalf horizontally to make 6 layers total.DO AHEAD: Cakes can be baked 1 day ahead.Wrap tightly in plastic wrap and store atroom temperature.

Stir chocolateand corn syrup in a large metal bowl setover a large saucepan of simmering wateruntil melted and smooth; remove from heat.Stir sour cream, salt, and vanilla in a mediummetal bowl; set over same saucepan ofsimmering water. Stir until mixture is warmbut not hot, about 2 minutes.

Add sourcream mixture to chocolate mixture; stiruntil smooth and glossy. Set aside.

Using an electric mixer, beat powderedsugar and butter in a medium bowl untillight and fluffy, 2–3 minutes. Scrape downsides of bowl. Slowly beat in chocolatemixture. Continue beating until no lumpsremain, about 2 minutes. Frosting shouldbe smooth and shiny. Cover and chill untilslightly firmed up, about 30 minutes.

Place 1 cake layer on a cake stand or largeplate. Spoon 1/3 cup frosting over; usingan offset spatula or the back of a spoon,smooth frosting to edge of cake, creating aneven layer.

Place another cake layer on top.Repeat with frosting and remaining cakelayers.

Spread remaining frosting over topand sides of cake. DO AHEAD: Cake can bemade 1 day ahead. Cover with a cake domeand store at room temperature.