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1
Whisk together both flours, cornstarch, and salt in a bowl.
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2
Put butter, sugars, lemon zest, and 1 tablespoon lemon juice into the bowl of an electric mixer fitted with the paddle attachment.
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3
Beat on medium speed until fluffy, about 3 minutes.
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4
Reduce speed to low.
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5
Add flour mixture in three batches, mixing well after each addition.
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6
Cover dough with plastic wrap; refrigerate 30 minutes.
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7
Preheat oven to 350F.
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8
Place cold dough between two pieces of parchment paper and roll out to 1/8 inch thick.
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9
Transfer dough on parchment to a baking sheet; freeze 10 minutes.
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10
Using a fluted 1 1/8-inch round cutter, cut out dough; transfer to parchment-lined baking sheets.
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11
Reroll scraps, and cut out (you should have 40 rounds).
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12
Bake until pale golden, 10 to 11 minutes, rotating sheets halfway through.
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13
Let cool slightly on sheets on wire racks.
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14
Transfer cookies to racks to cool completely.
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15
Using a rubber spatula, press the jam through a fine sieve into a small bowl.
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16
Stir in remaining tablespoon lemon juice.
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17
Spread 1 teaspoon jam mixture on flat side of half of the cookies, and sandwich with the remaining cookies.
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18
Dust with confectioners sugar.
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19
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.
