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

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

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

Add the wet ingredients to the dry ingredients and mix until just combined. The batter will be thick.

Carefully stir in the hot water. The batter will be thin.

Pour the batter evenly into the prepared cake pans.

Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.

While the cakes are cooling, make the frosting. In a large bowl, beat the softened butter until creamy.

Gradually add the cocoa powder and mix until well combined.

Alternately add the powdered sugar and milk, beating until smooth and creamy. Stir in the vanilla extract.

Once the cakes are completely cool, frost them with the chocolate frosting. Place one cake layer on a serving plate, spread a generous amount of frosting on top, then place the second cake layer on top and frost the entire cake.