Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, or line them with parchment paper to prevent sticking.

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

In another bowl, whisk the buttermilk, vegetable oil, eggs, and vanilla extract until smooth.

Slowly pour the wet ingredients into the dry ingredients and stir gently until just combined. Then, add the hot coffee (or hot water) and mix until the batter is smooth and slightly thin. This step enhances the chocolate flavor and keeps the cake moist.

Pour the batter evenly into the prepared cake pans. Bake in the preheated oven for 30–35 minutes, or until a toothpick inserted in the center comes out clean.

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

In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar, cocoa powder, and salt, mixing on low speed. Add the vanilla extract and heavy cream (or milk), one tablespoon at a time, until the frosting is smooth and spreadable.

Place one cake layer on a plate and spread a generous amount of frosting on top. Place the second layer on top and cover the entire cake with frosting using an offset spatula. Smooth out the frosting, or create decorative swirls if you like!