Step 1 — Preheat the Oven

Preheat your oven to 350 degrees F (175 degrees C). Position the rack in the center. Grease the sides and bottom of an 8-inch or 9-inch round cake pan generously with oil, then line the bottom with parchment paper for clean release.

Step 2 — Make the Buttermilk Mixture

In a small bowl or measuring cup, combine the milk and vinegar. Stir gently and set aside for 5 to 10 minutes until it curdles slightly. This tangy mixture activates the leavening agents and tenderizes the crumb.

Step 3 — Mix the Dry Ingredients

In a large mixing bowl, add the flour, cocoa powder, sugar, baking powder, baking soda, and salt. Whisk for 30 to 60 seconds to aerate and break up any cocoa clumps. Even distribution at this stage ensures a smooth, streak-free batter.

Step 4 — Add the Wet Ingredients

Pour the Greek yogurt, vegetable oil, vanilla extract, the prepared milk and vinegar mixture, and dissolved coffee if using directly into the dry ingredients. Stir gently with a spatula until no dry pockets remain. Mix only until just combined. Overmixing develops gluten and leads to a denser cake. Small lumps in the batter are completely normal.

Step 5 — Bake

Pour batter into the prepared pan and smooth the top lightly. Tap the pan firmly on the counter a few times to release trapped air bubbles. Bake for 30 to 35 minutes for an 8-inch pan or 28 to 32 minutes for a 9-inch pan. The cake is done when a toothpick inserted in the center comes out with a few moist crumbs and the top springs back lightly when pressed.

Step 6 — Cool

Place the pan on a wire rack and cool for 10 to 15 minutes. Run a thin knife around the edges, then invert onto the rack and cool completely for 1 to 2 hours before frosting or slicing.

Step 7 — Frost and Serve

Once fully cooled, frost with chocolate ganache, vanilla buttercream, or cream cheese frosting. For a layered birthday cake, level the tops before stacking. Serve at room temperature for best flavor and texture.