Ingredients

• 2 cups all-purpose flour
• 3/4 cup unsweetened cocoa powder (not Dutch-process) plus additional for dusting
• 1 1/4 teaspoons baking soda
• 3/4 teaspoon salt
• 2 sticks unsalted butter, softened
• 1 1/2 cups packed dark brown sugar
• 2 large eggs
• 1 teaspoon pure vanilla extract
• 1 1/3 cups water
• 2 large egg whites
• 1/2 cup sugar
• 1/4 cup light corn syrup
• 2 tablespoons water
• 1 teaspoon pure vanilla extract

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Instructions

1. Preheat oven to 350°F with rack in middle. Butter and flour an 8-inch square cake pan (2 inches deep).
2. Whisk together flour, cocoa powder, baking soda, and salt.
3. Beat together butter and brown sugar with an electric mixer until pale and fluffy. Add eggs 1 at a time, beating well, then beat in vanilla. Add flour mixture and water alternately in batches, beginning and ending with flour mixture and mixing until just combined.
4. Pour batter into cake pan and smooth top, then bake until a wooden pick inserted in center comes out clean, 45 to 55 minutes. Cool in pan on a rack 1 hour. Transfer cake to a cake plate.
5. Make frosting: Combine frosting ingredients with a pinch of salt in a metal bowl set over a saucepan of simmering water and beat with a handheld electric mixer at high speed until frosting is thick and fluffy, 6 to 7 minutes. Remove bowl from heat and continue to beat until slightly cooled. Mound frosting on top of cake. Dust with additional cocoa powder.
6. Combine frosting ingredients with a pinch of salt in a metal bowl set over a saucepan of simmering water and beat with a handheld electric mixer at high speed until frosting is thick and fluffy, 6 to 7 minutes. Remove bowl from heat and continue to beat until slightly cooled. Mound frosting on top of cake. Dust with additional cocoa powder.