First, wash the potatoes and add them to a pot of salted water. Bring the water to a boil and cook until they are soft enough to puncture with a fork.
Drain, peel, and mash the potatoes, either with a potato masher or a food mill. Transfer the mashed potatoes to a mixing bowl and add the egg yolks, Parmesan, a pinch of nutmeg, and salt and pepper to taste. Mix wellTIP: if the potatoes seem a little too thick you can always add a bit of milk to thin it out – be careful not to add too much though! Line a cake tin with parchment paper and then lightly oil. Add a layer of breadcrumbs on the bottom of the pan then add a few spoons of the mashed potatoes. Start compressing the potatoes down and adding more until you create a thick “crust” base. Don’t forget to get all the way up the sides of the tin, just like a pie crust, and make it thick but also leave enough potato for the top!
Add two layers of ham and then a layer of mozzarella on top of that, spacing everything out evenly through the cake. Add the rest of the mashed potatoes on top and compress it down until it covers the layers below and resembles a cake.
Sprinkle another layer of breadcrumbs on top then cut the butter into chunks and disperse evenly across the top.
Bake in the oven for 50 minutes at 390°. Let sit for 5-10 minutes before slicing up and serving.