Preheat the oven to 375° and line 2 baking sheets with parchment paper.
In a cup or small bowl, combine the water and flaxseed meal and allow to thicken for 3 to 5 minutes. In large bowl, whisk together the flour, baking powder, salt, and xanthan gum.
In the bowl of a stand mixer, cream together the shortening, sugars, and sunflower seed butter. Scrape down the sides of the bowl, then beat in the flaxseed mixture and the vanilla. Scrape down the sides of the bowl again. Stir in the dry ingredients until thoroughly combined.
Using a small ice cream scoop, drop the dough 2 inches apart onto the prepared sheets. Press the cookies down with the tines of a fork (dipped in sugar) in the criss-cross pattern characteristic of peanut butter cookies.
Bake the cookies for 10 to 12 minutes or until the edges are golden and the tops no longer look wet. Transfer the baking sheets from the oven to cooling racks and cool for 10 minutes, then transfer the cookies directly onto the racks to cool completely.
Store the cookies in an airtight container at room temperature for up to 5 days, or freeze for up to 3 months.
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Allergy-Free Desserts. Excerpted by permission of John Wiley & Sons, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.