Place the fully cooled baked cake layers on a clean work surface and trim their tops level if needed.
Tint 30 g of the stiff buttercream pale blue and reserve the remaining stiff buttercream white.
Crumb-coat the 4-inch cake with the softer buttercream and chill for 10 minutes.
Stack the 2-inch cake on the 4-inch cake using a dab of stiff white buttercream as glue. Frost the 2-inch tier with the blue buttercream and smooth the 4-inch tier with the remaining stiff white buttercream.
Add one tiny fondant star or sprinkle. Chill 15 minutes before photos.