Prep Dry Ingredients: In a medium bowl, whisk together the gluten-free flour blend, cornstarch (or arrowroot), baking powder, and salt. Set aside.
Cream Wet Ingredients: In a separate medium bowl, use an electric mixer to beat the slightly softened vegan butter and sugar until it's light and creamy, about 3 minutes. Scrape down the sides of the bowl as needed.
Add Flavor: Beat in the unsweetened dairy-free milk and vanilla extract until just combined.
Form Dough: Gradually add the dry flour mixture to the creamy butter mixture. Mix on low speed (or with a wooden spoon) until a soft, slightly sticky dough forms. Gather the dough into a ball.
Chill Dough (Crucial Step!): Flatten the dough ball into a disk, wrap it tightly in plastic wrap, and refrigerate for at least 3 hours (or up to 3 days). This prevents spreading!
Preheat & Prep: Once chilled, remove the dough from the fridge and let it sit at room temperature for 10-15 minutes to soften slightly, making it easier to roll. Preheat your oven to 350°F (177°C) and line baking sheets with parchment paper.
Roll Out Dough: Lightly dust a piece of parchment paper with a little gluten-free flour. Place the dough on top, then cover with another piece of parchment. Using a rolling pin, roll the dough evenly to about ¼-inch thickness.
Cut & Bake: Use your favorite cookie cutters to cut out shapes. Carefully transfer them to your prepared baking sheets, leaving about 1 inch between cookies. Gather dough scraps, gently form into a ball, and repeat rolling and cutting (if dough gets too warm and sticky, chill it for 15 minutes). If using, sprinkle with sanding sugar before baking.
Bake to Your Liking: Bake the cookies. For softer cookies, bake for 11-12 minutes. For crispier cookies, bake for 13 minutes. (Smaller cookies may need slightly less time.)
Cool: Remove from the oven and let cookies cool on the baking sheet for 1-2 minutes before carefully transferring them to a wire rack to cool completely. They must be fully cool before decorating!
To Make Optional Icing: In a small bowl, whisk together all icing ingredients until smooth. If too thick, add more milk ¼ teaspoon at a time. Add gel food coloring if desired. Use a piping bag with a small tip or a butter knife to decorate cooled cookies, then add sprinkles!
To Make Optional Vegan Buttercream: In a mixing bowl, beat softened vegan butter and salt with an electric mixer for 2 minutes until creamy. Gradually add powdered sugar on low speed, scraping the bowl. Add vanilla and milk (1 teaspoon at a time) until desired fluffy consistency. Beat on medium for 2-3 minutes. Add gel food coloring if desired. Spread or pipe onto cooled cookies, then add sprinkles!