Preheat your oven to 350°F (175°C). Grease two 8-inch round cake pans and line them with parchment paper to ensure easy removal after baking.

In a large bowl, sift together the cocoa powder, sugar, flour, baking soda, baking powder, and salt. Whisk until evenly mixed to avoid lumps.

In a separate bowl, whisk the buttermilk, vegetable oil, eggs, and vanilla extract. Slowly pour this mixture into the bowl with the dry ingredients, stirring until just combined.

Gradually add hot water to the batter while stirring gently. The mixture will be quite thin, which is essential for a moist and tender cake.

Pour the batter evenly into the prepared pans. Bake for 35–40 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for about 10 minutes before gently removing them and transferring them to a wire rack to cool completely.

In a microwave-safe bowl, melt the dark chocolate in short intervals, stirring frequently to avoid scorching. Set it aside to cool slightly. In a large mixing bowl, beat the butter and powdered sugar until light and creamy. Mix in the cocoa powder, followed by the sour cream, melted chocolate, and a small pinch of salt. Whisk until the frosting is silky and spreadable.

Position one cake layer on a serving platter and spread a generous layer of frosting over the top. Place the second layer carefully on top and cover the entire cake with frosting, smoothing the surface with a spatula. Add decorative swirls or patterns for a polished finish, if desired.