Step 1 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Preheat the oven to 350°F.
Step 2 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Grease a 9x13-inch pan with nonstick cooking spray. Line bottom of pan with parchment paper or dust with flour, shaking out any excess. Set pan aside.
Step 3 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Place the oil, brown sugar, granulated sugar, and vanilla in a stand mixer bowl or large mixing bowl. Blend with the paddle attachment (or hand mixer) on medium speed to combine, scraping the sides of the bowl as needed.
Step 4 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Add the eggs and yolks one at a time, blending after each addition and scraping the bowl as needed. Add the sour cream and milk to batter and beat to blend.
Step 5 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Measure the boiling water, then add the espresso powder and cocoa directly into the measuring cup and whisk until smooth.
Step 6 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Set a sheet of parchment paper (or a plate) on a work surface and sift the flour, baking powder, baking soda, and salt onto it, using a fine strainer or sifter. With the mixer on low speed, slowly pour half of dry ingredients into the wet. Blend in half of the cocoa mixture on low speed. Repeat with remaining dry ingredients and cocoa mixture, beating on low speed just to blend. Increase speed to medium and mix until dry ingredients are incorporated.
Step 7 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Thoroughly scrape the batter into the prepared pan. Batter will be thin.
PRO TIP Scrape the bowl well (wiping with a paper towel if needed), as you'll be reusing it to make the frosting.
Step 8 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Bake the cake on middle rack of oven for 20 minutes.
Step 9 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Rotate pan and continue baking until a toothpick inserted in center of cake comes out with a moist crumb or two, 20-25 minutes.
Step 10 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Check to see that cake is done. Remove from oven or add time as needed.
Step 11 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Let the cake rest on a cooling rack until cool enough to handle, about 30 minutes. Run a thin knife around the cake to loosen it from sides of pan, then carefully invert onto a serving platter or baking sheet to cool completely. Meanwhile, make the frosting.
PRO TIP If you don't have a platter big enough for the cake, a baking sheet works beautifully.
Step 12 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Place the butter in stand mixer bowl. Mix on medium speed until smooth, 1-2 minutes. Scrape down the sides of the bowl as needed.
Step 13 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Set a large sheet of parchment paper or a plate on a work surface and sift the powdered sugar and salt onto it. Use the paper to pour dry ingredients into the whipped butter a little at a time, alternating with a little cream and mixing on low speed to incorporate between each addition, until frosting is creamy and smooth.
PRO TIP If you have it, parchment paper is a handy tool for one-bowl baking, not only for lining a pan, but for using as a landing pad when sifting and transferring dry ingredients.
Step 14 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Add the vanilla and mix on medium-low speed until the frosting is light and fluffy, at least 5 minutes. Scrape down the sides of the bowl as needed. Cover the bowl and set frosting aside until cake is cool.
Step 15 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Tuck strips of parchment paper or waxed paper under the cake to keep the platter clean while you frost. Generously spread cooled cake with frosting, leaving sides bare if desired, and sprinkling with sparkling sugar if you want. Carefully pull out the strips of parchment.
PRO TIP Yes, that's a lot of frosting! For a cake that's a little less decadent, use as much frosting as you like, then refrigerate the rest in an airtight container for up to three weeks or freeze for up to two months.
Step 16 of 16
One-Bowl Chocolate Sheet Cake with Vanilla Buttercream
Cut cake into slices and serve. Store leftovers at room temperature loosely wrapped in plastic wrap or draped in parchment paper for up to three days.