Directions

1. Line 2 baking sheets with nonstick baking mats or parchment paper; set aside. Whisk together both flours, cornstarch, and salt in a bowl.
2. Put butter, sugars, lemon zest, and 1 tablespoon lemon juice into the bowl of an electric mixer fitted with the paddle attachment. Beat on medium speed until fluffy, about 3 minutes. Reduce speed to low. Add flour mixture in three batches, mixing well after each addition. Cover dough with plastic wrap; refrigerate 30 minutes.
3. Preheat oven to 350 degrees. Place cold dough between two 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 1/2-inch round cutter, cut out dough; transfer to prepared baking sheets. Reroll scraps, and cut out (you should have 40 rounds). Bake until pale golden, 10 to 11 minutes, rotating sheets halfway through. Let cool slightly on sheets on wire racks. Transfer cookies to racks to cool completely.
5. Place jam in a small bowl and 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. Unfilled cookies can be stored in an airtight container at room temperature up to 3 days; filled ones are best eaten the day you assemble them.