Think of these beautiful cookies as mini blooming versions of the classic linzer tart. They boast the same mouthwatering combination of buttery hazelnuts and fruity jam, just in a smaller package.
Pulse hazelnuts in a food processor until finely ground.
In the bowl of an electric mixer fitted with the paddle attachment, beat together butter and granulated sugar on medium speed until light and fluffy, about 2 minutes. Add egg and beat until smooth. Beat in vanilla.
Whisk together ground hazelnuts, flour, baking powder, cinnamon, salt, and nutmeg. Add to butter mixture; beat on low speed until just combined. Divide dough in half; shape each half into a flat disk and wrap in plastic. Refrigerate until firm, at least 1 hour or up to overnight.
Preheat oven to 350 degrees. Line two baking sheets with parchment. On a lightly floured surface, roll out one disk of dough 1/4 inch thick. Using a 3 1/2-inch flower-shaped cutter, cut out cookies. (If dough becomes too soft, freeze 10 minutes.) Using a large spatula, transfer to prepared baking sheets. Using a 3/4-inch petal-shaped cutter, cut out centers of petals from half the cookies. Repeat process with remaining disk of dough. Gather scraps from both disks, reroll, and cut.
Bake, rotating halfway through, until edges are slightly golden, 12 to 14 minutes. Transfer sheets to wire racks and let cool completely.
Lightly sift confectioners' sugar over cookies with cutout petals. Spread a heaping teaspoon of jam on each solid cookie; carefully sandwich with sugared cookies.