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1
Using back of spoon, mash lemon peel and salt to paste in medium bowl. Add butter. Using electric mixer, beat until fluffy. Add sugar in 4 batches, beating after each addition until blended.
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2
Boil lemon juice and lemon peel in small saucepan until mixture is reduced to 2 tablespoons, about 4 minutes. Pour into small bowl and cool. Sift flour, salt, and baking soda into medium bowl.
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3
Using electric mixer, beat butter in large bowl until fluffy. Add sugar; beat until blended. Beat in eggs, 1 at a time, then beat in yolks. Beat in lemon juice mixture. Add dry ingredients and stir to blend well. Cover dough and chill until firm, at least 4 hours and up to 1 day.
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4
Preheat oven to 375u00b0F. Line 2 baking sheets with parchment paper. Roll out 1/3 of dough on lightly floured surface to scant 3/8-inch thickness. Using 2-inch round cutter, cut out cookies. Transfer cookies to prepared baking sheets. Bake cookies, 1 sheet at a time, until golden at edges, about 12 minutes. Let cookies cool 5 minutes. Transfer to racks and cool completely. Repeat with remaining dough.
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5
Spread 2 teaspoons filling on flat side of 1 cookie. Top with another cookie, flat side down. Repeat to make more sandwich cookies. DO AHEAD:
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6
Store airtight in refrigerator.
