Make the dry mix: in a medium bowl whisk together gluten-free flour, baking powder, and salt. Set aside.
Cream butter and allulose: in a stand mixer fitted with the paddle attachment, beat softened grass-fed butter and allulose together on medium speed 3-4 minutes until pale, fluffy, and noticeably lighter in color. Allulose creams identically to granulated sugar - this step looks exactly as it should.
Add egg and vanilla: add the egg and vanilla extract to the butter mixture. Beat on medium until fully incorporated, about 1 minute. Scrape down the sides of the bowl.
Add flour: with the mixer on low, add the dry flour mixture gradually. Mix until just incorporated and the dough comes together - do not overmix. The dough will be slightly softer than a conventional sugar cookie dough.
Chill: shape the dough into a flat disc, wrap tightly in plastic wrap, and refrigerate for at least 1 hour. This step is not optional - chilled dough holds its shape when cut and baked. Warm dough spreads.
Preheat and prep: preheat oven to 300F. Note: reduced from the conventional 325F because allulose browns faster than granulated sugar. Line two baking sheets with parchment paper.
Roll and cut: remove dough from refrigerator. Place between two sheets of parchment paper and roll to ⅛-inch thickness. Cut with lightly floured cookie cutters and transfer to prepared baking sheets about 1 inch apart. Re-roll scraps once.
Bake: bake 13-16 minutes, rotating sheets halfway through. Watch carefully starting at 13 minutes. You want the edges genuinely golden - this is Grandma Beba's standard and it is the correct standard. Remove when the edges are golden and the centers look just set. They will firm as they cool.
Cool completely: let cookies cool on the baking sheet 5 minutes before transferring to a wire rack. Cool completely before decorating - minimum 30 minutes.
Decorate: with your kids, with your family, with whoever is in the kitchen. That is the point of this recipe.