To make the dough:

Cup of Yum
1. In a small bowl, sprinkle the yeast over the lukewarm milk. Stir briefly to combine and allow to sit until frothy, about 10 minutes.
2. In the bowl of a stand mixer fitted with a dough hook or a large bowl, combine the flour, sugar, and salt.
3. Mix in the frothy yeast with milk and egg until a soft dough comes together.
4. On a lightly floured surface, knead the dough until smooth and elastic. Transfer to a large bowl, cover, and refrigerate 8 hours to overnight.
5. Remove the chilled dough from the refrigerator and bring to room temperature for 30 minutes.
6. Cut the butter into 1/2 inch (1.25 centimeter) thick slices and arrange over a large piece of parchment in an 8x8 inch (20 x 20 centimeter) square.
7. Cover with another piece of parchment and hit the butter evenly with a rolling pin to bring the edges together, then roll into a 9x9 inch (23 x 23 centimeter) square.
8. Refrigerate the square of butter for 10 minutes, only enough to make it cool again but not completely solid. It should still be a little pliable without snapping.
9. On a lightly floured surface, roll the rested dough into a large 12x12 inch (30 x 30 centimeter) square.
10. Place the prepared sheet of butter in a diamond shape over the square with the corners placed towards the center of each dough edge.
11. Fold in the corners of the dough to meet in the center of the butter, pinching together the edges to fully enclose the butter.
12. Being careful to not crack the dough and expose the butter, roll the dough into a rectangle about 10x14 inches (25x36 centimeters) in size.
13. Fold 1/3rd of the dough inward towards the center followed by the other side to create three layers. Cover in plastic and refrigerate for 30 minutes.
14. Roll the dough out again and fold inwards again, then refrigerate another 30 minutes. Repeat the process two more times for a total of four times in all.

To assemble:

1. In a medium bowl, beat together the grated marzipan, softened butter, and sugar to create a smooth, sticky mixture.
2. Line two baking sheets with parchment or lightly grease.
3. In a small bowl, beat together the egg and milk.
4. Roll the rested dough into a large sheet about 18x12 inches (46x30 centimeters) in size on a lightly floured surface.
5. Cut the dough in half crosswise using a sharp knife to make two rectangles 9x12 inches (23x30 centimeters) in size.
6. Spread half of the marzipan filling down the center of one dough sheet lengthwise.
7. Fold 1/3 of the dough over the filling. Lightly brush the edge with the beaten egg milk mixture.
8. Fold the other 1/3 of the dough over the filling and folded dough, making sure about 1 inch (2.5 centimeters) are overlapping, and press to seal.
9. Flip the filled sheet over so the seam is on the bottom.
10. Using a sharp knife, cut the sheet into individual pastries about 2 inches (5 centimeters) wide crosswise or about 6 equal pieces.
11. Arrange on the prepared baking sheet at least 2 inches (5 centimeters) apart and cover the baking sheet with a towel.
12. Repeat with the other sheet of dough and marzipan to make 12 pastries in all.
13. Allow the covered pastries to rest at room temperature until puffed and doubled in size, 2-3 hours at room temperature.
14. Place the egg milk mixture in the refrigerator until ready to bake.

To bake:

1. Preheat oven to 400˚F (200˚C).
2. Brush the egg milk mixture over the top of each puffed pastry.
3. Cover completely with poppy seeds.
4. Bake in preheated oven for 5 minutes. Reduce heat to 375˚F (190˚C) and bake for another 10-15 minutes or until golden.
5. Allow to cool for about 5 minutes before removing from the baking sheet and enjoying or cooling on a wire rack.