Into a big mixing bowl, or in a bowl of a stand mixer, add the room temperature butter, lemon zest, and the sugar, and cream it for 2-3 minutes or until it’s creamy and light in color.
Add the egg yolk, kosher salt, and vanilla extract. Cream the butter until the mixture is creamy and light.
Add the flour and the almond flour. Mix until the mixture is just combined and you can’t see pockets of flour.
Cover the Linzer cookies dough with plastic wrap and let the dough rest in the fridge for at least an hour. We let the dough rest in the fridge overnight but it’s not a must.
After letting the dough chill in the fridge for at least an hour, take one dough half out of the fridge. Start rolling the dough on a floured clean surface to about ¼ inch thick.
Using cookie cutters, cut as many cookies as you can, we use a heart shape one, star one, and a circle one but you can use your favorite cookie cutters as well.
Then use small ½ inch cookie cutters like heart shape, circle, and a star shape! Carefully transfer the cookies onto a parchment paper-lined baking sheet and bake in a 180c (350f) preheated oven for 10 minutes or until the edges are starting to turn golden.
Take the cookies out of the oven and let them cool for 10 minutes. Then transfer them to a wire rack to cool completely.
Dust the cookies with the holes with powder sugar and set them aside. Place about ½ teaspoon of jam or your favorite filling on the center of each whole cookie and carefully top each cookie with the powdered sugar cookie and create a sandwich cookie.