Heat oven to 425°F; line 2 rimmed sheets with parchment. Cover potatoes with cold water by 1 inch; add cooking salt. Simmer 15 to 20 minutes after bubbling starts until very tender.
Drain thoroughly; dry in warm pan over low 2 minutes, gently shaking. Rice into a bowl; steam-dry 5 minutes.
Fold in 3 tablespoons melted butter, then yolks, seasoning salt, white pepper, and nutmeg until smooth enough to pipe. Do not whip or add milk.
Transfer warm mixture to a sturdy bag with large star tip. Pipe 24 equal mounds about 2 1/2 inches wide, spaced apart. Lightly brush tops with remaining butter.
Bake 20 to 25 minutes, rotating sheets halfway, until ridges golden and centers reach 160°F. Rest 3 minutes; serve 3 per person.