Place the potatoes in a pot of cold salted water, bring to a boil, and cook for 15-18 minutes until fork tender. Drain, return them to the pot, and stir over low heat for 1-2 minutes to dry them out, which keeps the piped shapes sharp.
Press the potatoes through a ricer or mash until perfectly smooth. Beat in the butter, warm oat milk, salt, white pepper, and nutmeg.Let the mixture cool for 5 minutes, then beat in the egg yolks one at a time until glossy.
Preheat the oven to 425°F and line a baking sheet with parchment paper. Spoon the potatoes into a piping bag fitted with a large star tip and pipe 2 inch wide swirls onto the sheet, spacing them about 1 inch apart.
Gently brush the swirls with the melted butter, taking care not to flatten the ridges.
Bake for 20-25 minutes until the edges and tips are deep golden brown. Sprinkle with chives and serve warm.