Preheat oven to 350ºF. Prepare two 4x2 square pans. If using larger pans, trim cakes to achieve this size.
In the bowl of a stand mixer, sift together the flour, cocoa powder, baking soda, baking powder and salt. Add the sugar and set aside.
Add the eggs, vanilla, oil and milk and beat on medium speed until well combined using the paddle attachment. Add champagne and continue beating on low until completely combined.
Using a wooden spoon, stir the boiling water into the batter. Pour the batter into each pan. I weigh my batter to achieve identical baked cakes.
Bake for 25-30 minutes, or until a tooth pick inserted into the centre comes out clean.
To make the buttercream:
Using the paddle attachment of your stand mixer, cream together the sugar and butter. Beat in the vanilla beans. Add the champagne one tablespoon at a time, until desired consistency is achieved. Beat until smooth.
Assembly:
Level cakes if needed. Fill with a generous amount of buttercream and stack. Mask the cake with the remaining buttercream. Add gold flakes to sides, finishing up with the top. Add topper.