Place the flours into a mixing bowl, then add the xylitol and a pinch of salt. Crumble the butter into the mixture with your hands by rubbing it between your palms together with the flour. If the butter is too hard, grate it using a cheese grater to make it easier to work with. It is very important that the fat coats the flour particles well, because this is what makes your dough crumbly and tender.
Add the egg yolk and the grated lemon zest. Work the dough together. Try to work quickly, because if you are too slow, the dough can easily become greasy. You will notice this when it becomes harder and harder to shape, as the butter starts melting from the warmth of your hands.
Shape the finished dough into a ball, wrap it in plastic wrap to prevent it from drying out, and place it in the refrigerator for about 40 minutes.
After 40 minutes, prepare your rolling pin and board. Dust the surface with flour to make rolling the dough easier.
Take the dough out of the fridge and knead it briefly by hand again. At first, it will feel hard, but the warmth of your hands will slightly soften the butter, making it easier to shape.
Roll out the dough to about ½ cm thickness.
Cut out shapes as desired. I personally like heart shapes, but you can even use a glass as a cutter. Make sure your linzer cookies have both a top and a bottom layer. You can also cut a smaller shape into the top pieces so the jam will show nicely.
Place the cookies onto a baking tray lined with parchment paper. If the dough becomes too sticky (especially during the second rolling), use a dough scraper to lift the pieces and transfer them onto the tray. You can place them close together, but do not let them touch completely, as they will expand slightly while baking.
Bake in a preheated fan-assisted oven at 180 ℃ (top and bottom heat) for 15 minutes.
Allow the cookies to cool completely, then spread jam onto the bottom pieces and place the tops on them.