Put the butter and eggs outside the fridge so it can get to room temperature. Zest the lemons and set aside.
Add flour, cornstarch, baking powder, and salt to a bowl and whisk. Set aside. Add butter and granulated sugar into the bowl of your electric mixer (use the paddle attachment). Mix on medium-high speed until fluffy. Mix in egg yolks, vanilla extract and lemon zest. Reduce speed to low, and gradually mix in the flour mixture.
Once the dough looks firm enough get it out of the bowl and finish my kneading it with your hands. Shape the dough into a disk and wrap in plastic. Let it rest in the fridge for 1 hour.
Preheat oven to 175 degrees. Line a baking sheet with baking paper. Slightly flour your work surface and add the dough to it. Roll out to a generous 3 mm thick dough. Use your favorite Christmas mold to cut out the shapes. Re-use the scraps, by rolling it again and cut out.
Add the pieces carefully to the baking sheets. I used a a little help of my cake-server. Space 2 inches apart on baking sheets. Refrigerate until firm for about 15 minutes.
Bake until light golden for about 12 minutes. Let the cookies cool completely on the baking sheets.