Dissolve yeast and a teaspoon of sugar in warm milk; let it become frothy.

In a bowl, combine flour, sugar, and salt; add yeast mixture and egg, mixing into a dough.

Knead the dough until smooth and elastic, about 5 minutes.

Cover and refrigerate dough for 30 minutes to rest.

Flatten chilled butter between parchment sheets into an 8x10-inch rectangle.

Roll out the dough into a 12x20-inch rectangle; place butter on half and fold over.

Roll out dough and fold into thirds; wrap and chill for 30 minutes.

Repeat rolling and folding two more times, chilling between each fold.

After final fold, refrigerate dough for 2 hours or overnight.

For filling, mix ground nuts, powdered sugar, egg white, and vanilla until paste-like.

Roll out dough to 1/4 inch thickness and cut into triangles.

Add filling (if using) and roll each triangle from base to tip.

Shape into crescents and let rise for 1 hour.

Preheat oven to 400°F (200°C).

Whisk egg and milk for egg wash; brush over Gipfeli.

Bake for 18-20 minutes until golden brown.

Cool slightly and serve warm.