For the Lemon Curd:
In a medium, heavy, nonreactive saucepan, combine the eggs and egg yolks, lemon zest, lemon juice, sugar and butter. Bring to a full boil, whisking constantly, over medium-high heat. Boil for 1 minute, whisking constantly; it will be fairly thin. Remove from heat; whisk for 30 seconds more. Let stand until cooled to warm. Strain the lemon curd through a fine sieve into a medium bowl. Cover tightly and refrigerate until chilled and thickened, at least 1 1/2 hours. (The lemon curd may be refrigerated for up to 4 days.)
For the dough:
Preheat the oven to 350 degrees F. Grease several baking sheets or coat with nonstick spray. In a medium bowl, thoroughly stir together the flour, Clabber Girl Baking Powder, baking soda and salt; set aside. In a large bowl, with an electric mixer on medium speed, beat together the butter, sugar, and lemon zest until very light and fluffy. Add the egg yolk, 1 tablespoon water, the vanilla and lemon juice and beat until thoroughly incorporated. Reduce the speed to low and beat in half the flour mixture. Gradually beat or stir in the remaining flour mixture until well blended.
Shape portions of the dough into generous 1 1/4-inch balls with lightly greased hands. Place on the baking sheets, spacing about 2 1/2 inches apart. Using your thumb or knuckle, press a deep well into the center of each ball; take care not to break through the dough, or the lemon curd will leak out during baking. Place about 3/4 teaspoon lemon curd in each well, mounding it slightly.
Bake the cookies, one sheet at a time, in the middle of the oven for 10 to 13 minutes, or until the lemon curd is bubbly and the cookies are lightly browned at the edges. Reverse the sheet from front to back halfway through baking to ensure even browning. When cookies are done, transfer cookie sheet to a wire rack and let stand until the cookies firm up slightly, 1 to 2 minutes. Using a spatula, transfer the cookies to wire racks. Let stand until completely cooled.
Store in an airtight container for up to 2 days. The cookies may be frozen for up to 1 month but will be soft rather than crisp.
From: The All-American Cookie Book by Nancy Baggett