Preheat the oven to 350˚ F. Butter a 9-by-13-inch baking pan (for a thinner cake, use a 13-by-18-inch sheet pan). Dust the bottom and sides of the pan with cocoa powder.
Whisk 2 1/4 cups flour, 1 1/4 teaspoons baking soda, 3/4 teaspoon salt and 1/2 teaspoon baking powder in a bowl.
Pour 1 cup boiling water over 3/4 cup Dutch-process cocoa powder in a separate bowl; whisk until smooth. Let cool 10 minutes, then whisk in 2 cups granulated sugar, 1/2 cup each vegetable oil and buttermilk, 2 room-temperature eggs and 1 tablespoon vanilla. Whisk in the flour mixture in two batches until smooth.
Spread the batter in the prepared pan and tap on the counter to release any air bubbles. Bake until a toothpick inserted into the center comes out clean, 35 to 45 minutes (if using a sheet pan, bake only 30 to 35 minutes). Transfer to a rack and let cool completely in the pan before frosting.