Adapted from American Cookie by Anne Byrn. Reprinted with permission from Rodale/Penguin Random House, 2018.
½ cup unsalted butter at room temperature
½ cup plus 2 teaspoons sugar
3 large eggs at room temperature and separated
1 scant cup (4 ounces) flour, plus more for rolling dough
⅛ teaspoon salt
⅛ teaspoon ground nutmeg
⅓ cup powdered sugar
½-¾ cup fruit jam (such as strawberry, blueberry, or fig), or lemon curd
In a large mixing bowl, beat butter and sugar with an electric mixer on medium-high speed until light and creamy, about 1 minute. Add egg yolks, one at a time, beating well after each addition. Set aside.
In a small bowl, whisk together flour, salt, and nutmeg. Add flour mixture to butter mixture, beating on low speed until just combined, about 30 seconds.
Turn dough out onto a sheet of plastic wrap and wrap tightly. Place in refrigerator to chill until firm, at least 2 hours.
Preheat oven to 375 degrees and position a rack in center. Scatter a couple tablespoons flour on a clean surface and turn out chilled dough. Generously flour a rolling pin and roll dough out to ¼-inch thickness.
Using a 2½-inch cookie cutter, cut rounds and place on ungreased baking pans about 2 inches apart. Bake until cookies are lightly browned around the edges, 8 to 10 minutes. Transfer cookies to a rack to cool and repeat with remaining dough.
Once all dough is baked, increase oven temperature to 400 degrees. Line two baking pans with foil and set aside.
In a large mixing bowl, beat egg whites on high speed until soft peaks form, about 2 minutes. With mixer on high, add powdered sugar a little at a time and beat until stiff and glossy peaks form, about 1 minute more. Set aside.
Arrange cooled cookies on baking pans. Dollop about 1 teaspoon of jam in the center of each cookie, followed by about 2 heaping tablespoons of meringue on top. Using a small metal spatula, spread meringue to edges of each cookie (to seal meringue to the cookie so it doesn't shrink while cooling).
Bake the cookies, one pan at a time, until meringues are lightly browned, about 7 to 8 minutes. Using a spatula, carefully transfer warm cookies to a rack to cool. The cookies are best when served immediately, but they’ll keep at room temperature for 2 days.