In a large pot, cover potatoes with cold salted water by 2 inches. Bring to a boil; reduce to a rapid simmer and cook until tender, 15 minutes. Drain in a colander, let sit 5 minutes, then return to pot. Add butter and, with a potato masher, mash until smooth. Season with salt and pepper. Add 2 egg yolks, 1/4 cup cream, and pinch of nutmeg; stir until combined.
Preheat oven to 450 degrees. Line a baking sheet with parchment and lightly butter paper. Dollop mixture in 8 equal portions on sheet, 2 inches apart. Using the back of a spoon, create a small well in the center of each. Freeze until firm, 15 minutes. Whisk together remaining egg yolk and 2 teaspoons cream. Brush on potatoes and bake until golden, 15 minutes.
These mounds of soft, buttery spuds with crisp golden crusts have indented centers made to hold pools of gravy.