Prepare the Oven and Cake Pan:
Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or line it with parchment paper.
Mix Dry Ingredients:
In a large mixing bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt.
Combine Wet Ingredients:
In another bowl, whisk together the eggs, oil, buttermilk, brewed coffee, and vanilla extract until smooth and well-combined.
Combine Wet and Dry Ingredients:
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Avoid overmixing.
Bake the Cake:
Pour the batter into the prepared cake pan and bake for 30 minutes or until a toothpick inserted into the center comes out clean.
Cool the Cake:
Let the cake cool in the pan for 10 minutes, then transfer it to a cooling rack to cool completely.
Prepare the Frosting:
In a mixing bowl, beat the butter with an electric mixer until smooth. Gradually add the powdered sugar and cocoa powder, mixing until combined. Add brewed coffee 1 tablespoon at a time to adjust the consistency of the frosting. Stir in vanilla extract.
Frost the Cake:
Once the cake has cooled, spread the frosting evenly over the top and sides of the cake using a spatula.
Serve and Enjoy:
Slice the cake and enjoy your moist, rich chocolate coffee-flavored treat.