Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt until well combined.
In another bowl, mix the buttermilk, vegetable oil, eggs, and vanilla extract until smooth. Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Slowly stir in the hot water until the batter is smooth and well blended. The batter will be thin.
Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring them to wire racks to cool completely.
While the cakes are cooling, prepare the frosting. In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar and cocoa powder, mixing until combined. Add the heavy cream and vanilla extract, and beat until the frosting is light and fluffy.
Once the cakes are completely cool, place one layer on a serving plate. Spread a layer of frosting on top, then place the second layer on top and frost the top and sides of the cake.