Activate yeast & mix dough: In a small bowl, whisk warm milk, yeast, and 1 tsp sugar; let sit 5 min until foamy. Whisk in egg, remaining sugar, and vanilla.

Build dough: In a stand mixer bowl, combine flour and salt. Add milk mixture and knead on medium speed 5–6 min until smooth and elastic. Form into a disk, wrap, and chill 30 min.

Prepare butter block: On parchment, roll butter into a 6×6″ square. Chill until firm but pliable (about 10 min).

First turn: Roll dough to 10×10″. Place butter diagonally in center; fold dough corners over to seal. Roll to a 12×8″ rectangle; fold into thirds like a letter. Wrap and chill 30 min.

Repeat turns: Perform three more roll-fold-chill cycles (total of four turns), always chilling 30 min between turns.

Shape cronuts: Roll final dough to ½″ thickness. Use a 3″ cutter for outer ring and a 1″ cutter for the hole. Place shapes on a floured tray, cover lightly, and proof 1 hr at room temperature until puffy.

Fry cronuts: Heat oil to 350°F. Fry cronuts in batches, 2–3 min per side, until deep golden and puffed.

Coat & fill: Drain on paper towels. While warm, toss in cinnamon-sugar. If filling, use a piping bag to inject pastry cream or jam.