Whisk flour, cornstarch, baking powder, and salt in a medium bowl. Put butter and granulated sugar into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-high speed until pale and fluffy. Mix in yolks, lemon zest, and vanilla. Reduce speed to low, and gradually mix in flour mixture. Shape into a disk; wrap in plastic. Refrigerate until firm, about 1 hour.
Preheat oven to 350. Transfer dough to a lightly floured work surface. Roll out to a generous 1/8 inch thick. Using a 2 1/2-inch round fluted cutter, cut out wreaths. Reroll scraps, and cut out. Cut out centers of wreaths using a 7/8-inch round cutter. Space 2 inches apart on baking sheets lined with parchment paper. Refrigerate until firm, about 15 minutes. Bake until just golden, about 12 minutes. Let cool completely on wire racks.
Stir together confectioners' sugar and lemon juice in a small bowl. Dip surface of each cookie in icing, and sprinkle with pistachios. Let set on wire racks.
Cookies can be stored in airtight containers at room temperature up to 1 week.