Gluten-Free Citrus Sugar Cookies

Arrange the racks in the upper and lower rungs in the oven and preheat the oven to 325 °F. Use a coffee grinder or mortar and pestle to grind the psyllium, flax and chia seeds to a fine powder, transfer to a bowl along with the flour mix, cornstarch, and salt. Whisk a few times to combine, and set aside. In a separate large bowl, beat the sugar and butter together using an electric mixer. Beat in the yolks and vanilla until combined. Add the flour mixture and mix just until combined. Divide the dough in three equal parts, placing each in a medium bowl. Add one type of zest to each bowl. Mix the zest into the dough until evenly distributed. Form each dough into a disc, wrap in plastic, and refrigerate until dough has firmed up a bit, about 15 minutes. Place each piece of dough between two pieces of parchment and roll to a 8-x-6-inch rectangle. Stack the dough sheets and transfer to the refrigerator. Chill until firmed up, about 30 minutes. Remove the dough from the refrigerator and remove top layers of parchment. Using a sharp knife, cut the dough lengthwise and crosswise to form 1 1/4-inch squares. Arrange the dough squares 2 inches apart on two baking sheets. Chill until firm, about 15 minutes. Bake cookies until light golden on the edges, about 15 minutes. Let cool for 10 minutes on the baking sheet, then transfer the cookies to wire rack set over a parchment-lined baking sheet to cool completely.