In a large bowl, use an electric mixer to beat the softened butter and sugar together for about 2 minutes, until the mixture is light and fluffy. Add the egg and vanilla extract, then continue beating until fully incorporated
In a separate small bowl, whisk together the flour, baking powder, cream of tartar, and salt.
Gradually add the dry ingredients to the butter mixture, beating on low speed until everything is completely combined and a soft dough forms.
Once the dough is mixed, lightly knead it for about 1 minute until smooth. Shape it into a log (about 2 inches in diameter), wrap tightly in plastic wrap, and refrigerate for at least 1 hour (or overnight) to firm up.
When ready to bake, preheat your oven to 350°F and line two baking sheets with parchment paper or silicone baking mats.
Spread a generous amount of red sugar crystals onto a dinner plate. Remove the chilled cookie log from the refrigerator, unwrap it, and roll the entire log in the sugar crystals to coat the outside evenly.
Using a sharp knife, slice the log into ½-inch thick rounds. For extra sparkle, gently press the top and sides of each sliced cookie into the remaining sugar crystals to ensure they’re fully coated.
Arrange the coated cookie slices on the prepared baking sheets, leaving a little space between each one.
Bake in the preheated 350°F oven for 8-12 minutes, or until the edges are just lightly golden (be careful not to overbake for the best soft texture)
Remove from the oven and let the cookies cool on the baking sheets for a few minutes before transferring them to the cooling wire racks to cool completely.
Serve these dazzling Red Glitter Cookies as a delightful afternoon snack, perfect party favors for Valentine’s Day, or a standout addition to your holiday gatherings and Christmas cookie exchanges.