How to Make It
Beat butter, sugar, and zest at medium speed with an electric mixer 1 minute. Add egg and egg yolk; beat 30 seconds until blended. Scrape bowl; beat 30 seconds.
Pulse flour and next 3 ingredients in a food processor until finely ground; gradually add flour mixture to butter mixture, beating until blended.
Shape dough into 2 (1/2-inch-thick) rectangles. Wrap each rectangle in plastic, and chill 4 hours to 3 days.
Preheat oven to 350°F. Generously flour both sides of dough; place on parchment paper. Roll each into a 14- x 10-inch rectangle. Cut each rectangle into 24 squares with a lightly floured 2-inch scalloped square cutter, rerolling scraps as needed. Chill on parchment paper 30 minutes.
Place cookies 1 inch apart on parchment paper-lined baking sheets. Cut centers out of half of cookies with a lightly floured 1 ¼-inch square- or flower-shaped cutter. (If desired, place dough centers on a parchment paper-lined baking sheet; chill 15 minutes, and bake as directed.)
Bake, in batches, at 350°F for 12 to 14 minutes or until golden brown around edges. Cool on parchment paper on a wire rack.
Spread ¼ tsp. jam onto each solid cookie; top with hollow cookies.