Heat oven to 350°F. Grease two 9-inch round cake pans with butter or cooking spray. Dust the inside of pans with a small amount of cocoa powder, not flour — cocoa disappears into the crumb while flour leaves a white film on dark cake. If making a sheet cake instead, use a 9×13-inch pan and grease only, no dusting needed.

In a large bowl, whisk together 2 cups all-purpose flour (measured by spooning into cup and leveling off, not scooping), 2 cups sugar, ¾ cup unsweetened cocoa powder, 2 teaspoons baking soda, 1 teaspoon baking powder, and 1 teaspoon salt. Measure flour by volume carefully or use a kitchen scale for 250g — over-packed flour causes dry cake.

In a separate bowl or large measuring jug, whisk together 2 large eggs, 1 cup buttermilk, ½ cup vegetable oil, and 2 teaspoons vanilla extract until smooth. Ensure all wet ingredients are at room temperature — cold eggs or buttermilk can cause the batter to look slightly broken before baking.

Pour the wet ingredients into the dry ingredients and mix on low speed or stir by hand just until combined, about 30 seconds. Stop as soon as you no longer see dry streaks. Do not overmix — overmixing develops gluten, which tightens the crumb and makes the cake chewy rather than tender.

Pour in 1 cup of hot coffee or hot water and stir to combine. The batter will be noticeably thin and shiny, much thinner than most cake batters — this is correct and intentional. Do not add more flour.

Divide the batter evenly between the two prepared 9-inch round pans using a kitchen scale if possible so each pan holds roughly equal weight. For a 9×13-inch sheet cake, pour all batter into the single pan.

Bake for 30–35 minutes for layer cakes, or 38–42 minutes for a 9×13-inch sheet cake. The cake is done when a toothpick inserted in the center comes out with just a few moist crumbs — not wet batter, but not bone dry. An instant-read thermometer should read 205–210°F. Underbaking by even 3–4 minutes is better than overbaking, which dries out the cake.

Let the cake layers cool in the pans for 10 minutes, then turn out onto a wire rack to cool completely before frosting. Do not frost warm cake — it will melt the frosting and cause uneven crumb.