1. In a medium bowl, sift together the flour, cornstarch, cinnamon, and salt. Set aside.
2. In the bowl of a stand mixer, beat together the butter, lemon zest, and lemon oil (if using) until the mixture becomes light in color. Add the sugar and beat to combine.
3. Next, beat in the egg on medium high, followed by the egg yolk and vanilla or almond extract. Beat until light and fluffy (about 2 to 3 minutes), then reduce speed to low. Gradually add the flour mixture, mixing until just incorporated.
4. Form the dough into a ball. Divide the ball into thirds. Wrap each piece in plastic wrap and refrigerate for a minimum of 1 hour. This dough can also be refrigerated overnight.
5. Preheat the oven to 350 °F. Lightly dust a work area with flour. Place one piece of dough on the floured work surface. Roll out the dough to ⅛-inch thickness. Using a 3-inch round cookie cutter, cut out circles. Consolidate any scraps and repeat until all the dough is used.
6. Set aside half of the circles. With the other half, use a smaller cookie cutter or a knife to cut out a smaller design in the center (such as a heart, star, teardrop, or smaller circle). Transfer all of the cookies to a baking sheet lined with parchment paper and refrigerate for 10 minutes.
7. Bake for approximately 10 minutes or until the edges of your cookies are golden in color. Remove your cookies from the oven and allow them to cool on the cookie sheets for 8 to 10 minutes, then transfer to cooling racks. Continue rolling out and baking the remaining dough.
8. After the cookies are completely cool, spread jam over the rounds WITHOUT a cut-out.
9. Using a sifter or a fine mesh sieve, lightly dust the rounds WITH cut-outs with powdered sugar.
10. Assemble the cookies by placing the powdered sugar round on top of the jam round. They should look like little jam sandwiches. You can store your cookies in an airtight container in the refrigerator if you don’t eat them all right away!

Notes

Inactive Time: Rest for 1 hour to overnight