Sieve 100g sugar and 300g flour together, combine with 200g cold butter cut into pieces, and one egg yolk.
Knead together until one rather sticky glob of dough. (It will at first seem as if all those ingredients never could stick together - and eventually becomes so it seems as if more flour might be necessary.)
Let rest in the fridge for a while. (30 minutes)
Take out the ball of dough, knead it to warm it a bit (cold, it would just break apart and/or be too solid to work), flour the baking board/surface and the dough well, and roll the dough out to about 5 mm thickness. It will be necessary to take good care the surface underneath the dough and the surface of the dough (or the roller) are well-dusted with flour, or the dough will just stick and crumble apart. Even so, some tendency for the dough to break must be expected…
Cut out circles, with a cookie shape if you have it, but a stronger glass of the desired width does the trick very well, too. Put the cut-out cookies onto parchment/baking paper on baking trays. Knead together remaining dough and cut out further cookies, repeat until dough is used up.
Bake the cookies for about 15 minutes at 180C; they should get golden-brown but not burnt-brown - let the thinner cookies be your guide. When done, take out of the oven and let cool well. (Do not try to hurry things and lift the cookies off when they are still warm or they'll just break and crumble!