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.
Roll out dough to an 11-inch square. Cut 6 tablespoons 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 wrap, and refrigerate 30 minutes.
Place dough with a short side facing you and opening on 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 30 minutes. Roll out dough again; repeat process with remaining 6 tablespoons butter.
Filling and topping: Preheat oven to 400 degrees. Stir together butter and sugar until smooth. Roll out dough on a lightly floured surface to an 11-by-16-inch rectangle. Spread butter mixture on bottom two-thirds of dough. Fold top one-third of dough over, then fold over again to make three layers.
Place dough with a long side facing you. Brush top with egg, then sprinkle with both seeds to cover top, 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. 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 deep golden, rotating once, about 18 minutes. Let cool completely on sheets on wire racks before serving.