These triangular shaped cookies are usually eaten around the Jewish holiday Purim, but are delicious anytime of year!
WHIRL flour, sugar, poppy seeds, lemon zest, baking powder and salt in a food processor. Add butter and pulse until coarse crumbs form. Whisk yolks, egg and vanilla in a small bowl and pour through feed tube, pulsing until dough starts to come together. Gently knead into a ball and divide in half. Wrap in plastic wrap and press into discs. Refrigerate at least 1 hour or up to 3 days (bring to room temperature before rolling).
PREHEAT oven to 350F. Line 2 baking sheets with parchment. Roll dough on lightly floured surface to scant ¼-in. thickness. Use a 3-in. round crinkle cookie cutter to cut out dough, rerolling scraps. Scoop a scant 1 tsp jam into centre of each round. Lightly brush edges of rounds with milk. Pinch in 3 corners to seal, forming a triangle, leaving jam centre open. Refrigerate for 15 min. Bake in top and bottom thirds of oven, rotating and switching sheets halfway, until pastry is golden, 17 to 18 min. Let cool slightly. Transfer to a rack to cool completely. Cookies will keep well in a covered container at room temperature for up to 2 days.