Refrigerating the glazed cakes will help them set more quickly, but they will lose some of their shine. If there's time and you prefer a shiny glaze, let the cakes set at room temperature.
Preheat oven to 350 degrees. Butter two 8-inch round cake pans and dust with cocoa, tapping out excess. Sift cocoa, flour, sugar, baking soda, baking powder, and 3/4 teaspoon salt into the bowl of a mixer. Beat on low speed until just combined.
Raise speed to medium, and add eggs, buttermilk, warm water, oil, and vanilla. Beat until smooth, about 3 minutes.
Divide batter between pans. Bake until set and a toothpick inserted into center of each comes out clean, about 35 minutes. Let cakes cool in pans set on wire racks for 15 minutes. Turn out cakes. Let cool completely.
Transfer 1 cake to a wire rack set on a rimmed baking sheet. Pour half the warm ganache over top; pour in center, then slowly but steadily pour around edges, allowing ganache to drip down sides and cover cake. Let stand for 15 minutes. Transfer cake to a cake stand. Repeat with remaining cake and ganache.
Let cakes stand until set, about 3 hours at room temperature or 1 hour in the refrigerator.
Unglazed cakes can be stored at room temperature, wrapped in plastic wrap, for up to 2 days. Glazed cakes can be stored at room temperature for up to 1 day.