1. In a bowl, sift together the flour, cornstarch and salt.
2. In another bowl, using an electric mixer on high speed, beat together the butter and lemon zest until light.
3. Add sugar and beat until completely incorporated.
4. Add whole egg and egg yolk and beat until fluffy.
5. Reduce speed to low, add flour mixture and beat until just incorporated.
6. Gather dough into a ball and divide into thirds.
7. Flatten each third into a disk.
8. Wrap separately in waxed paper and refrigerate for at least 1 hour or overnight.
9. Preheat oven to 350°F.
10. Lightly grease baking sheets.
11. Dust 1 dough disk with flour and place between 2 large sheets of waxed paper.
12. Roll out dough to 1/8 inch thick.
13. Using a 2 1/2-inch decorative cutter, cut out cookies.
14. Transfer cookies to prepared baking sheets, spacing them about 1/2 inch apart.
15. Refrigerate for 10 minutes.
16. Gather dough scraps into a ball, wrap in waxed paper and refrigerate.
17. Bake until edges of the cookies are golden, about 10 minutes.
18. Transfer cookies to wire racks and let cool.
19. Repeat with the remaining 2 disks, then roll out, cut, chill and bake scraps in the same manner.
20. On half of the cookies, spread jam on the bottoms, spreading it lightly at the edges.
21. Place the remaining cookies on top of the jammed cookies, making little sandwiches (with the bottoms adhering together with jam).
22. Dust with powdered sugar.
23. Store in an airtight container in the refrigerator for up to 1 week.

Makes about 3 dozen cookies.

Idea: To create a cute heart cutout, use a small heart-shaped cookie cutter to cut a heart on half of the cookies before baking, so you can see the jam through the cutout when you assemble the cookies.

Tip: Always start with chilled dough and a well-floured work surface. Chilled dough does not soften as quickly as warmer dough; flour absorbs any moisture and prevents sticking.

Approximate Nutrient Analysis per cookie:
140 calories, 4 g fat, 2.5 g saturated fat, 20 mg cholesterol, 25 mg sodium, 24 g carbohydrate, 0 g fiber, 14 g sugar, 1 g protein