Boil lemon juice and lemon peel in small saucepan until mixture is reduced to 2 tablespoons, about 4 to 5 minutes. Pour into small bowl and cool.
Sift flour, salt, and baking soda into medium bowl, and set aside. Using electric mixer, beat butter in large bowl until fluffy. Add sugar to butter, and beat until blended. Beat in eggs, 1 at a time, then beat in yolks. Beat in lemon juice mixture and lemon extract. Add dry ingredients and stir to blend well. Cover dough and chill until firm, at least 4 hours and up to 2 days; store airtight in refrigerator.
Preheat oven to 375°F. Line 2 baking sheets with parchment paper or Silpats. Roll out 1/3 of dough on lightly floured surface to scant 3/8-inch thickness. Using 1 to 1.5 inch round cutter, cut out cookies. Transfer cookies to prepared baking sheets. Bake, 1 sheet at a time, until golden at edges, about 6 minutes. Let cookies cool 5 minutes. Transfer to racks and cool completely. Repeat with remaining dough.
Using back of spoon, mash lemon peel and salt to paste in medium bowl. Add lemon extract and lemon juice, and mix to combine. Add butter, and using an electric mixer, beat until fluffy. Add sugar in batches, beating well after each addition.
Spread 2 teaspoons filling on flat side of 1 cookie. Top with another cookie, flat side down. Repeat to make more sandwich cookies.
Makes about 36 sandwiches
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