• In the bowl of a stand mixer or a large bowl with a hand mixer, cream together the butter, granulated sugar, and powdered sugar until light and fluffy, about 2 minutes.
  1 cup unsalted butter, ½ cup granulated sugar, ½ cup powdered sugar
• Add the egg and vanilla extract and mix until fully combined, scraping down the sides as needed.
  1 large egg, 1 teaspoon vanilla extract
• In a separate bowl, whisk together the flour, baking soda, cream of tartar, and salt. Gradually add the dry ingredients to the wet ingredients and mix just until combined.
  2½ cups all-purpose flour, ½ teaspoon baking soda, ½ teaspoon cream of tartar, ½ teaspoon salt
• Divide the dough into two equal portions. Place each portion between two sheets of parchment paper and roll into a rectangle about ¼-inch thick. Flip the parchment once or twice while rolling to keep the thickness even.
• Transfer the rolled dough (still between parchment) to a baking sheet and freeze for 15 minutes, or until firm but not rock hard.
• Preheat the oven to 350°F and line a baking sheet with parchment paper.
• Lightly flour a clean work surface. Peel the parchment from the chilled dough and place it on the surface. Lightly dust the top with flour and cut into shapes using cookie cutters.
• Gather scraps, re-roll to ¼-inch thick, and cut again. If the dough becomes soft, return it to the freezer for 10–15 minutes.
• Transfer cookies to the prepared baking sheet, spacing about 1 inch apart. Bake for 8–11 minutes, just until the edges look set and barely start to turn golden.
• Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before decorating.

Icing (optional)

• Mix powdered sugar, milk, and vanilla until smooth. Adjust with a little more milk as needed to reach your preferred consistency. Add food coloring if desired.
  3 cups powdered sugar, 2-3 Tablespoons milk, ½ teaspoon vanilla extract, Food coloring
• Spread icing over completely cooled cookies and add sprinkles while the icing is still wet.
  Sprinkles