2 cups all-purpose flour, plus more for dusting
1/2 teaspoon baking powder
1/4 teaspoon salt
1/2 cup unsalted butter, at room temperature
1/2 cup vegetable shortening
1 cup granulated sugar
1 large egg, separated
1 tablespoon finely grated lemon zest
2 teaspoon fresh lemon juice
1 bunch fresh thyme, leaves and tips picked
In a bowl, whisk together the 2 cups flour, the baking powder, and the salt. In a large bowl, using an electric mixer on medium-high speed, beat the butter, shortening, and granulated sugar until light and fluffy, 2—3 minutes.
Add the egg yolk and lemon zest and juice and beat on low speed until the egg is completely incorporated.
Beating on low speed, slowly add the flour mixture and continue to beat just until incorporated, scraping down the sides of the bowl as needed. Press the dough into a 7-inch (18-cm) disk.
Wrap tightly in plastic wrap and refrigerate until firm, at least 1 hour or up to overnight.
Preheat the oven to 325°F (165°C). Line 3 baking sheets with parchment paper. In a small bowl, whisk the egg white with 2 teaspoons water.
On a floured work surface, using floured rolling pin, roll dough to 1⁄4 inch (6 mm) thick. Using a 21⁄2-inch (6-cm) round cookie cutter, cut out shapes. Use a spatula to transfer cookies to prepared sheets, spacing them 1 inch apart.
Use a 1-inch round cookie cutter to cut a small circle from the center of each shape. Press the dough scraps together, roll out, and cut out additional wreaths. Brush each wreath with reserved egg white mixture, top with thyme leaves and sanding sugar.
Bake until the cookies are lightly golden around the edges, 14 - 17 minutes. Let cool on the sheets for 5 minutes.
Using the spatula, transfer the cookies to wire racks and let cool, about 30 minutes. Spoon the icing into a paper cone with a 1⁄8-inch (3-mm) round tip (page 15). Pipe dots of icing on each wreath. Let the icing dry completely, about 2 hours.
Store the cookies in an airtight container, layered between sheets of parchment paper, in the refrigerator for up to 4 days.