Fudgy Chocolate Sheet Cake

Preheat oven to 325°F. Line the bottom of two 9×13 rectangular cake pans with parchment paper. Grease parchment and sides of pan.

Using an electric mixer, beat butter with sugar in a large bowl until light and fluffy.

Add eggs, one at a time, beating each in until just incorporated and scraping down sides of bowl. Add vanilla, then buttermilk. (Don’t worry if the batter looks uneven and grainy; it will all be fine in a minute.)

Place flour, cocoa powder, baking soda, baking powder, and salt in a sifter over the mixing bowl and sift ingredients in.

Beat or stir dry ingredients into batter until just combined; scrape down the bowl again.

Divide batter between two pans. Bake each layer for 30 to 40 minutes, rotating them once top-to-bottom and back-to-front halfway through. Cakes are ready when a toothpick inserted into the center of each layer comes out batter-free.

Cool in the pan 10 minutes, then remove cakes from pans and let cool completely on racks. Freeze to firm up this very soft cake before leveling and frosting it.

To make fudge buttercream filling, place all ingredients in a food processor and pulse to incorporate, then process until the frosting is smooth.

Remove first cooled cake layer from the freezer. Place on the cake board or serving tray. Arrange small scraps of waxed or parchment paper underneath edges, to protect the plate from frosting as you decorate.

Level the top of the cake if it has domed by shaving off the dome with a horizontal serrated knife. Spread fudge buttercream filling evenly over the first cake layer.

Place second layer on top of filling; return the cake to the freezer while you make the frosting.

To make vanilla buttercream, beat the butter until light and fluffy in a large bowl. Add sugar 1/2 cup at a time until fully incorporated. Beat in half-and-half then vanilla.

Thinly coat the entire cake with frosting (this is a “crumb” coat), then return the cake to the freezer for 10 minutes, to set the first layer of frosting, then finish frosting the cake with the remaining frosting.

Tint and pipe any decorations needed with remaining buttercream. If making the day before the party, keep it in the fridge overnight. An hour or so at room temperature should be sufficient to defrost.