Preparation

Make the meringues

• Position racks in the top and bottom thirds of the oven and heat the oven to 175°F. Line 2 large heavy-duty baking sheets with parchment.

  In a food processor, process the sugar until very fine, about 45 seconds. Add the confectioners’ sugar and salt and pulse until well blended, about 15 seconds.

  In a stand mixer fitted with the whisk attachment, combine the egg whites and cream of tartar. Begin mixing on medium-low speed until frothy. Increase the speed to medium high and beat until the whites form soft peaks, about 2 minutes. Continue beating while gradually sprinkling in the sugar mixture. When all the sugar is added, increase the speed to high and whip until firm, glossy peaks form, about 10 minutes. Add the vanilla and lemon extracts and beat just until blended, about 10 seconds.

  Spoon the meringueinto a large pastry bagfitted with a plain 3/32-inch tip (Ateco #3). Holding the pastry bag perpendicular to a lined baking sheet, pipe small flat disks about 1-1/4 inches in diameter and no higher than 1/4 inch, spaced about 1/2 inch apart. As you pipe, keep the tip down into the meringue and let it spread out rather than lifting the tip

  up. If necessary, you can help the meringue spread by circling the tip in the meringue as you pipe. Sprinkle the crushed cookies evenly over the tops of the meringues. Don’t worry if the extra crumbs fall off. Using a fingertip, lightly press down on the crumbs to flatten any meringue peaks.

  Bake the meringues until dried and crisp but not browned, about 2-1/2 hours. Turn off the oven and leave the door shut; let the meringues sit in the oven until cool, about 2 hours. Remove from the oven and gently lift the meringues off the parchment. (At this point, you can assemble the cookies or store the meringues in an airtight container for up to 1 month.)

Make the lemon curd

• Melt the butter in a 4-quart saucepan over medium heat. Remove the pan from the heat and whisk in the sugar, lemon juice, zest, and salt. Whisk in the yolks until well blended. Cook over medium-low heat, whisking constantly, until the mixture is thick enough to coat a spatula and hold a line drawn through it with a finger, 4 to 6 minutes. Don’t let the mixture boil.

  Strain the curd into a clean bowl and cover with plastic wrap, pressing the plastic onto the surface to keep a skin from forming. Let cool at room temperature; then refrigerate until chilled. (The curd will keep for up to 3 weeks.)

Assemble the cookies

• Arrange half of the meringues bottom side up on a work surface. Put the chilled curdin a piping bagfitted with a plain 3/8-inch tip (Ateco #7) and pipe about 1 tsp. of curd onto each meringue. Gently press the remaining meringues bottom side down onto the curd.

  The filled cookies can be prepared and kept at room temperature up to 1 hour before serving. The longer they sit, the softer the meringues become.