Preheat the oven to 350°F (175°C). Grease two 8-inch or 9-inch round cake pans and line the bottoms with parchment paper.

In a large bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.

In a separate bowl, whisk together the eggs, buttermilk, vegetable oil, and vanilla extract.

Gradually add the wet ingredients to the dry ingredients, mixing just until combined.

Slowly pour in the hot coffee or hot water. Mix carefully until the batter is smooth. The batter will be fairly thin.

Divide the batter evenly between the prepared cake pans.

Bake for 28–35 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Let the cakes cool in their pans for about 10 minutes, then transfer them to a wire rack to cool completely.

To make the frosting, beat the softened butter until creamy.

Gradually add the powdered sugar and cocoa powder, mixing until combined.

Add the vanilla, salt, and enough milk or cream to reach a smooth, spreadable consistency.

Beat the frosting for another 1–2 minutes until light and fluffy.

Place one cooled cake layer on a serving plate and spread frosting over the top.

Add the second cake layer and frost the top and sides of the cake.

Slice and serve once assembled.