Steps:

Dough: Combine warm milk, sugar, and yeast; let sit until bubbly. Add oil, yolks, vanilla, flour, and salt; mix until smooth. Knead on floured surface for 5 mins. Cover and let rest 10-15 mins.

Custard Filling: In a saucepan, combine sugar and cornstarch, add milk and yolk. Boil for 1½ mins while stirring, then chill.

Shape & Bake: Roll dough ½-inch thick, cut circles, place on greased sheet, and let rise 1 hour. Bake at 350°F for 10-12 mins.

Coat & Fill: Brush warm donuts with melted butter, roll in sugar, then fill with custard or jam.

Enjoy your paczki!