Ingredients
1 1/2 sticks unsalted butter, room temperature, plus more for pan
1/2 cup boiling water
1/3 cup natural cocoa powder
3 ounces semisweet chocolate, finely chopped (1/2 cup)
2 cups unbleached all-purpose flour
1 teaspoon baking powder
1/2 teaspoon baking soda
1 teaspoon kosher salt
1 1/2 cups sugar
2 large eggs, room temperature
2 teaspoons pure vanilla extract
1 cup sour cream
1/2 recipe Vanilla Swiss-Meringue Buttercream
Chocolate curls, for serving (optional; see note, above)
Preparation
Preheat oven to 350 degrees. Butter a 9-by-13-inch baking pan. Line bottom with parchment; butter parchment. Stir together boiling water, cocoa, and chocolate; let cool 10 minutes. Meanwhile, whisk together flour, baking powder, baking soda, and salt in another bowl.
Beat butter with sugar on medium-high speed until fluffy, 2 to 3 minutes. Beat in eggs, one at a time, beating to combine after each addition and scraping down sides of bowl as needed. Beat in vanilla, then chocolate mixture. Reduce speed to low; beat in flour mixture in two batches, alternating with sour cream, until just combined. Scrape batter into prepared pan, smoothing top with an offset spatula.
Bake until a tester inserted in center comes out with a few moist crumbs, 35 to 40 minutes (top will not spring back). Let cool in pan 10 minutes. Invert onto a wire rack and remove parchment; let cool completely. Cake can be made up to 1 day ahead and stored at room temperature, wrapped in plastic.
Spread buttercream evenly over top and sides of cake. Sprinkle with chocolate curls. (Leftovers can be refrigerated, wrapped in plastic, up to 3 days; return to room temperature before serving.)
