Chocolate Fluff Cake

If you love cupcakes, this one is for you. It's really just a giant cupcake. Yummy-enjoy.

2 cups all-purpose flour
3/4 cup unsweetened cocoa powder (not Dutch-process)**
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
Preheat oven to 350°F with rack in middle. Butter and flour an 8-inch square cake pan (2 inches deep). Whisk together flour, cocoa powder, baking soda, and salt. 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.
Pour batter into cake pan and smooth top, then bake until a wooden pick inserted in center comes out clean, 45 to 55 minutes.
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.