How to Make It
In a food processor, combine the polenta, 3/4 cup of flour, confectioners' sugar, baking powder and salt and pulse to blend. Add the butter, shortening and lemon zest and pulse until the mixture resembles coarse sand. Add the egg and poppy seeds and pulse just until the dough forms a ball. Pat the dough into a disk and wrap in plastic; refrigerate for at least 1 hour, or until firm.
Preheat the oven to 350°. Generously flour a work surface and rolling pin. Roll out the dough 1/8 inch thick. Using a floured 2 1/4-inch round biscuit cutter, stamp out 28 cookies as close together as possible. Gather the dough scraps together and gently reroll the dough, then stamp out 4 more cookies. Carefully transfer the cookies to 2 ungreased baking sheets.
Bake the cookies on the upper and lower racks of the oven for 18 minutes, or until golden; shift the pans from top to bottom and front to back halfway through baking to ensure even browning. Let the cookies cool on the baking sheets.
To form the sandwiches, scoop about 1/4 cup of the softened sorbet onto each of 16 cookies. Top the sorbet with the remaining cookies, pressing lightly to help evenly spread the sorbet. Set the sandwiches on a baking sheet and freeze for about 1 hour, or until the sorbet is firm.