Preheat your oven to 350°F (175°C). Grease and flour four 8-inch round cake pans. In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt until well combined.

In a separate bowl, combine buttermilk, vegetable oil, eggs, and vanilla extract. Pour the wet ingredients into the dry ingredients and mix until just combined. Gradually add the boiling water while mixing on low speed until the batter is smooth.

Divide the batter evenly among the four prepared pans. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Cool the cakes in the pans for 10 minutes, then transfer to wire racks to cool completely.

Beat the softened butter until creamy. Add melted dark chocolate and mix until combined. Gradually add powdered sugar, heavy cream, vanilla extract, and salt. Beat on high speed for 3-4 minutes until light and fluffy.

Place the first cake layer on a serving plate. Spread with a generous layer of frosting. Repeat with remaining layers, ending with a final cake layer. Frost the top and sides of the cake with the remaining frosting.