This is a video of baker Rosanna Pansino demonstrating how to make a Mario question mark cake with gold coins inside (those gold foil coins with chocolate inside, not real gold coins, although the process would probably be the same except without me swinging through your kitchen on a rope with a cutlass between my teeth trying to steal the cake). The instructions don't really seem that difficult (recipe and text instructions on her blog HERE) provided you're familiar with cake baking and icing and fondant. If you aren't familiar with those thing then it will probably turn out like a dumpster fire and make its rounds across the internet next to a picture of this Mario question mark cake with the caption 'DELETE UR PINTEREST'.
Keep going for the video.
Thanks to Damien, who agrees the best cakes are the ones with surprises inside. Particularly a dancer who pops out to sing you Happy Early Retirement.