Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans.
In a large bowl, whisk together the flour, cocoa powder, baking soda, salt, and sugar.
In a separate bowl, whisk together the eggs, vegetable oil, and buttermilk.
Add the wet ingredients to the dry ingredients and mix until just combined. Stir in the hot water until the batter is smooth.
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, prepare the frosting. In a large bowl, beat the softened butter until creamy. Gradually add the cocoa powder and powdered sugar, alternating with the milk, beating until smooth and fluffy.
Stir in the vanilla extract.
Once the cakes are completely cool, frost the top of one cake layer. Place the second cake layer on top and frost the top and sides of the entire cake.
Notes
Decorate with fresh berries or chocolate shavings for an extra festive touch.