1. Preheat the oven to 350°F. Grease two 8-inch round cake pans, line the bottoms with parchment paper, and lightly dust the sides with cocoa powder or flour.

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

3. In a separate bowl, whisk the eggs, sour cream, vegetable oil, buttermilk, and vanilla extract until smooth.

4. Pour the wet ingredients into the dry ingredients and mix gently until mostly combined.

5. Add the hot coffee or hot water and stir until the batter is smooth and pourable.

6. Divide the batter evenly between the prepared pans and smooth the tops.

7. Bake for 28 to 32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

8. Cool the cakes in the pans for 10 minutes, then turn them out onto a wire rack to cool completely.

9. Beat the softened butter until creamy, then add cocoa powder, powdered sugar, heavy cream, vanilla extract, and salt. Beat until smooth and fluffy.

10. Place one cake layer on a serving plate, spread frosting over the top, add the second layer, then frost the top and sides. Slice and serve at room temperature.