Bloom the Dutch-process cocoa powder by mixing it with hot water or hot coffee.

Whisk together the dry ingredients, including the cake flour, in a large bowl.

Beat the wet ingredients together, incorporating the room temperature eggs and buttermilk.

Combine the wet and dry ingredients to form a batter, pour into pans, and bake until a toothpick comes out clean.

Let the sticky chocolate cakes cool completely on a wire rack.

Prepare the vanilla buttercream frosting, dividing it to dye the majority bright pink and a small amount green.

Frost the cake with the pink frosting, intentionally leaving it messy and imperfect just like Hagrid's.

Use a piping bag and the green frosting to write 'Happee Birthdae Harry' on top of the cake.