2 lbs (about 4–5 medium) Yukon Gold or Russet potatoes, peeled and cut into chunks
4 tbsp unsalted butter
½ cup heavy cream (or whole milk for a lighter version)
3 large egg yolks
½ tsp salt (adjust to taste)
¼ tsp black pepper
⅛ tsp nutmeg (optional but traditional)
1–2 tbsp grated Parmesan cheese (optional, for richer flavor)
1 beaten egg (for brushing)