1. Prep: Preheat oven to 350°F (177°C). Grease and flour two 8-inch round cake pans (or line with parchment).2. Mix Dry: In a large bowl, whisk together the flour, sugar, sifted black cocoa powder, baking soda, and salt.3. Mix Wet: In a medium bowl, whisk together the buttermilk, melted butter, eggs, vanilla, and vinegar.4. Combine: Pour the wet ingredients into the dry ingredients. Whisk until just combined (do not overmix).5. Bake: Divide batter evenly between the pans. Bake for 28-32 minutes, or until a toothpick inserted in the center comes out clean.6. Cool: Let cakes cool in pans for 15 minutes, then invert onto a wire rack to cool completely.7. Make Frosting: In a large bowl, beat the softened butter with an electric mixer until light and fluffy (about 3-4 minutes).8. Add Dry: Sift the powdered sugar and black cocoa powder. On low speed, gradually add the dry mixture to the butter, alternating with the heavy cream and vanilla. Add the salt.9. Whip: Once combined, increase speed to medium-high and beat for 3-5 minutes until fluffy. Add black gel food coloring if desired for a deeper black.10. Assemble: Level the cooled cakes. Place one layer on a plate, top with 1 cup of frosting. Add the second layer.11. Frost: Cover the entire cake with a thin crumb coat and chill for 20 minutes. Apply the final layer of black frosting.