Ingredients

• 1 1/2 tablespoons finely grated lemon peel
• 1/8 teaspoon salt
• 1/2 cup (1 stick) unsalted butter, at room temperature
• 2 cups (packed) powdered sugar
• 1/2 cup fresh lemon juice
• 1 tablespoon finely grated lemon peel
• 3 3/4 cups all purpose flour
• 1 teaspoon salt
• 1/2 teaspoon baking soda
• 1 cup (2 sticks) unsalted butter, room temperature
• 1 1/2 cups sugar
• 2 large eggs
• 2 large egg yolks

Instructions

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.
2. For cookies: 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. 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. Preheat oven to 375°F. 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. 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: Can be made 2 days ahead. Store airtight in refrigerator.
3. 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.
4. 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.
5. Preheat oven to 375°F. 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.
6. 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: Can be made 2 days ahead. Store airtight in refrigerator.