• Preheat the oven to 375°F/190°C.
• Whisk together the flour, cream of tartar, baking soda, and salt in a large mixing bowl and set aside.
• Using a mixer, beat sugar and butter in a large bowl until light and fluffy. Scrape the sides of the bowl with a rubber scraper as needed.
• Add the eggs and beat until smooth and creamy, scraping the sides of the bowl with a rubber scraper as needed.
• Mix in the vanilla.
• With the mixer on low, gradually add the dry ingredients to the wet ingredients in three batches until just combined. The dough will be thick.
• Shape the dough into 1¼-inch balls. At this point, you can roll them in sugar or sprinkles or move on to the next step if you're making iced cookies.
• Place them 2 inches apart on an ungreased cookie sheet.
• For a flatter cookie – press the dough down slightly before baking with your palm or the bottom of a glass. For a puffier cookie, leave the balls intact.
• Bake for 8 to 10 minutes or until set. Remove from cookie sheet to wire rack to cool.

For Iced Cookies

• Whisk together the ingredients for the icing, adding extra milk as needed, 1 teaspoon at a time, until desired consistency (a bit of milk goes a long way, so try to keep it slightly thick.)
• Spoon the icing on top of the cooled cookies.
• If using sprinkles, add them immediately to the wet icing. Let the icing harden before serving. You can store cookies for up to five days in an airtight container.