Step 1

Place a rack in middle of oven; preheat to 375°. Line the bottom of a 13x9" metal baking pan with parchment paper. Lightly coat paper and sides of pan with nonstick vegetable oil spray. Vigorously whisk 2 cups (400 g) granulated sugar, 1¾ cups (219 g) all-purpose flour, and 1 tsp. baking soda in a medium bowl to combine.

Step 2

Whisk ¾ cup (64 g) Dutch-process cocoa powder, ¾ cup vegetable oil, and 1 cup hot tap water in a large bowl until cocoa is fully dissolved, about 30 seconds. Add 2 large eggs, ¾ cup well-shaken buttermilk, 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt, and 2 tsp. vanilla extract and whisk again just to combine.

Step 3

Add dry ingredients to cocoa mixture and whisk gently until just combined and batter is smooth. Scrape batter into prepared pan.

Step 4

Bake cake until it springs back when gently pressed or a tester inserted into the center comes out with a few moist crumbs attached, 25–30 minutes. If serving cake directly from pan, let cool in pan, about 1 hour. If you prefer to serve cake unmolded, let cool in pan 10 minutes. Invert onto a wire rack and let cool completely. Peel away parchment; invert cake, right side up, onto a platter.

Step 5

Cook ¼ cup (½ stick) unsalted butter, room temperature, in small skillet over medium-low heat until it begins to foam; add ⅓ cup (32 g) nonfat dry milk powder and cook, stirring constantly with a heatproof rubber spatula, until mixture turns a dark nut brown color (milk powder will clump; that’s okay), about 5 minutes. Remove from heat and immediately add ⅔ cup heavy cream (this will prevent further cooking); stir vigorously until smooth (some small flecks may remain and that is okay). Let brown butter mixture cool, about 30 minutes.

Step 6

Using an electric mixer on medium-high speed, beat 2½ tsp. Diamond Crystal or 1½ tsp. Morton kosher salt, 1 tsp. vanilla extract, and remaining 1 cup (2 sticks) unsalted butter, room temperature, in a medium bowl until smooth and creamy, about 1 minute. Add 8 oz. powdered sugar, sifted, and beat, starting on low speed, until well combined, about 1 minute. Add brown butter mixture and remaining 8 oz. powdered sugar, sifted, and beat until frosting is light and fluffy, about 2 minutes.

Step 7

Spread frosting evenly over top of cake, working all the way to the edges. Using a spoon, create decorative divots and swirls in frosting if desired. Sprinkle with flaky sea salt (if using) and sprinkles.Do ahead: Frosting can be made 2 days ahead. Cover and chill. Bring to room temperature, then beat until fluffy again before using.