Peel the potatoes and cut into fairly equal sizes. Boil in a large pot until tender. Drain in a colander allowing to steam off all the moisture and shaking every now and then.
Using a potato ricer (if you have one) mash the potato into a large bowl for an electric mixer. (If you don't have a potato ricer or an electric mixer just use a masher and make sure there are no lumps).
Add the butter, salt, nutmeg and cream. Mix with the mixer for approximately 5 minutes until really smooth.
Add the egg yolks and mix on low until just combined. Do not overmix.
Using a large piping bag with a star tip pipe the potatoes into high mounds. I piped them on to baking paper in the containers I would place into the freezer.
If you're not going to freeze overnight I would recommend freezing for at least an hour.
Preheat oven to 200C°. Mix the egg and milk well.
Remove potato from freezer and place on a large tray, on baking paper. Brush with egg and milk mix - go lightly with this you don't want it to gather at the bottom as the stacks will collapse.
Bake in oven for 30 minutes.
