Preheat oven to 350°F (175°C). Grease and flour your cake pan or line it with parchment paper. Combine dry ingredients: In a large mixing bowl, sift together the flour, cocoa powder, baking soda, baking powder, salt, and sugar.

Add in the egg, vegetable oil, buttermilk, and hot water. Mix well using a whisk or electric mixer until smooth and lump free. Pour the batter into the prepared cake pan and smooth the top.

Bake for 28–32 minutes, or until a toothpick inserted into the center comes out clean. Cool completely on a wire rack before applying the frosting. In a medium bowl, beat the softened butter until creamy.

Add the cocoa powder and mix until combined. Gradually add powdered sugar, alternating with milk, until the desired consistency is achieved. Once the cake has cooled, frost the top and sides evenly using a knife or spatula.