Preheat your oven to 350°F (175°C). Grease two 8-inch round cake pans with butter or cooking spray and line the bottoms with parchment paper, greasing the parchment as well.

In a large mixing bowl, sift together the flour, black cocoa powder, white granulated sugar, baking powder, baking soda, and salt. Whisk until well combined.

In another bowl, mix the canola oil, vanilla extract, eggs, melted chocolate, and buttermilk. Whisk until smooth. Optionally, add black food dye.

Gradually add hot water to the wet mix, then slowly incorporate the dry mix, stirring until just combined. Avoid overmixing.

Divide the batter into the prepared pans and bake for 30 to 34 minutes, or until a toothpick comes out clean. Cool for 10 minutes in pans before transferring to a wire rack.

While the cakes cool, sift together the powdered sugar and black cocoa powder. Beat butter until fluffy, then gradually mix in the sugar-cocoa blend. Add vanilla, salt, and food dye if using.

Once cooled, frost one cake layer on a serving plate, add the second layer on top, and apply a light crumb coat of frosting. Refrigerate for about 10 minutes.

After chilling, frost the entire cake with a thicker layer of the black cocoa buttercream. Decorate as desired with themed decorations.