Directions

1. Preheat oven to 350 degrees. Whisk flours, cornstarch, and salt in a medium bowl.
2. Put butter, sugars, lemon zest, and 1 tablespoon lemon juice into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium speed until fluffy, about 3 minutes. Reduce speed to low. Add flour mixture in 3 batches, mixing well after each addition.Cover dough with plastic wrap; refrigerate 30 minutes.
3. Place cold dough between 2 pieces of parchment paper, and roll out to 1/8 inch thick. Transfer dough on parchment to a baking sheet; freeze 10 minutes.
4. Using a fluted 1 7/8-inch round cutter, cut out dough; transfer to parchment-lined baking sheets. Reroll scraps, and cut out (you should have 40 rounds). Bake until pale golden, 10 to 11 minutes. Let cool slightly on sheets on wire racks. Transfer cookies to racks to cool completely.
5. Using a rubber spatula, press the jam through a fine sieve into a small bowl. Stir in remaining tablespoon lemon juice. Spread 1 teaspoon jam mixture on flat side of half of the cookies, and sandwich with the remaining cookies. Dust with confectioners' sugar.