1. Line several baking sheets with parchment paper, or grease them lightly with vegetable oil.

2. Stir egg yolks and vanilla together using a fork in a small cup.

3. Place flour, sugar, salt and lemon zest in a food processor and process for 5 sec.

4. Scatter the butter over the flour and process until the mixture resembles coarse meal, 20-30 sec.

5. With the machine running, add the yolk mixture through the feed tube and process for 5 sec.

6. Scrape the sides and process until the dough comes together, 10-15 sec.

7. Scoop out rounded teaspoons of the dough and roll them into balls with your hands.

8. Place them 2" apart on the prepared baking sheets.

9. Using the bottom of a glass that has been lightly dipped in flour, flatten each ball to a generous 1/8" thickness. About 2.25" in diameter.

10. Bake cookies until the edges are deep golden, 14-16 min. Allow them to cool on baking sheets.

11. When the cookies are completely cool, turn half of them upside down.

12. Spread each bottom half with a generous 0.5 tsp. of preserves and top with the remaining cookies.

13. Sift XXX sugar over the tops.