Place the butter for the block on parchment, flatten it into a slab with your hands, wrap it in a parchment envelope, and roll it out thin with a rolling pin. Chill overnight.

Activate the yeast: add half a teaspoon of sugar to the warm water, stir in the yeast, and let sit for 15 minutes - it should get foamy.

Sift the flour into a bowl, add the salt and remaining sugar, and mix. Pour in the activated yeast and warm milk, and knead the dough.

At the end, add the softened butter (50 g) and knead into a smooth, pliable dough. Shape into a rectangle and chill overnight.

On a floured surface, roll the dough into a rectangle twice the size of the butter block, place the butter on one half, and fold the other half over it.

Roll out into a long, even slab. Fold the top third down and the bottom third up, so you end up with three layers. Wrap in plastic and chill for 1 hour.

Roll into a long slab again, fold both edges toward the center, wrap in plastic, and chill for another hour.

Roll the dough into a rectangular slab and cut into triangles with a knife. Notch the center of each triangle's base and roll up the croissant, stretching the dough gently as you go, into a crescent shape.

Place the shaped croissants on parchment, cover with plastic wrap, and let them proof and rise well.

Brush with beaten egg and bake for 10 minutes at 200°C, then lower to 190°C and bake for another 10-12 minutes. Let cool.