• In a small bowl, sprinkle the yeast over the lukewarm milk. Stir briefly to combine and allow to sit until frothy, about 10 minutes.
• In the bowl of a stand mixer fitted with a dough hook or a large bowl, combine the flour, sugar, and salt.
• Slowly pour the frothy yeast with the milk into the flour mixture, then the egg until a soft dough comes together.
• If still too crumbly, slowly add a little more milk. If too sticky to handle, add a little more flour.
• On a lightly floured surface, knead the dough until smooth and elastic. Transfer to a large bowl, cover, and rest at room temperature until doubled in size, about 1 1/2 hours.
• During the last 30 minutes of rest, cut the butter into 1/2 inch (1.25 centimeter) thick slices and arrange over a large piece of parchment in an 8×8 inch (20 x 20 centimeter) square.
• Cover with another piece of parchment and hit the butter evenly with a rolling pin to bring the edges together, then roll into a 9×9 inch (23 x 23 centimeter) square.
• 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.
• On a lightly floured surface, roll the rested dough into a large rectangle about 18 x 10 inches (46 x 25 centimeters).
• Place the prepared sheet of butter over 1/2 of the dough, keeping the edges clear.
• Fold the dough over in half to cover the sheet of butter, pinching together the edges to seal. Wrap the dough in plastic and refrigerate for 30 minutes.
• Being careful to not crack the dough and expose the butter, roll the dough on a lightly floured surface into a rectangle again about 16 x 10 inches (41 x 25 centimeters).
• 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.
• Roll the dough out again and fold inwards again, then refrigerate another 30 minutes. Repeat the process one more time for a total of three times in all. Refrigerate for another 30 minutes after the final folding.

To assemble the Gipfeli:

• Line two rimmed baking sheets with parchment or lightly grease.
• Using a sharp knife, cut the dough in half. Cover one half in plastic and place back in the refrigerator until you are ready to use it.
• For the remaining half, roll on a lightly floured surface into a long rectangle about 18 x 6 inches (46 x 15 centimeters).
• Cut the dough into triangles with a 5 inch (13 centimeter) base. There should be about 6 in all.
• Make a small perpendicular 1 inch (2.5 centimeter) cut in the center of each base (this will help keep the dough from puffing too much in the center and assist in stretching the ends into a crescent shape).
• Roll the triangle up, base to corner, stretching as needed, and slightly turn the ends inward to form a crescent. Lightly press down on the end to seal into the dough.
• Place on the prepared baking sheet and repeat with remaining triangles placing them 2 inches (5 centimeters) apart, then remaining dough to make around 12 Gipfeli total.
• Cover the baking sheets lightly with towels and allow to rest at room temperature for 1 hour, until puffed.
• Preheat oven to 400˚F (200˚C).
• In a small bowl, whisk together the egg yolk and 1 tablespoon (15 milliliters) milk to make the egg wash.
• Brush the tops of the rested Gipfeli with the egg wash and bake for 5 minutes. Decrease temperature to 375˚F (190˚C) and continue to bake until puffed and golden brown, another 10-12 minutes.
• These Gipfeli are best served warm from the oven and within a few of hours of baking with butter and jam.