For the Dough
For Filling and Topping
Make the dough: Whisk yeast into milk in a small bowl until dissolved. Add egg, sugar, and salt. Gradually stir in flour. Turn out dough onto a lightly floured surface and knead until smooth and supple, about 5 minutes.Advertisement
Roll out dough into an 11-inch square. Cut 6 tablespoons of butter into very thin slices and place on bottom two-thirds of dough. Fold top one-third of dough over (like a business letter), then fold over again to make three layers. Transfer to a baking sheet, drape with plastic, and refrigerate for 30 minutes.
Place dough with a short side facing you and opening on the right, like a book. Roll out dough to a 10-by-15-inch rectangle. Cover bottom two-thirds with 6 tablespoons butter, fold into thirds as in step 2, and refrigerate for 30 minutes. Roll out dough again; repeat process with remaining 6 tablespoons butter.
Add filling: Preheat oven to 400 degrees. Roll out dough on a lightly floured surface to an 11-by-16-inch rectangle. Spread 8 tablespoons softened butter on bottom two-thirds of dough and sprinkle with sea salt. Fold top one-third of dough over, then fold over again to make three layers.
Add topping: Place dough with a long side facing you. Brush top with egg, then sprinkle with poppy seeds to cover, gently pressing seeds into dough. Cut dough in half crosswise, then cut each half into 8 slices. Twist each slice, pulling slightly to form a tight spiral or a loose knot. Place on parchment-lined rimmed baking sheets, pressing on ends to secure shape. Drape with a kitchen towel and let rise in a warm spot until puffed, about 15 minutes.
Bake, one sheet at a time, until a deep golden brown, about 18 minutes, rotating once halfway through. Let cool completely on sheets on wire racks before serving.