Cream the butter and sugar: In a mixing bowl, beat softened butter and 3/4 cup sugar until light and fluffy, about 2–3 minutes. This traps air and helps the cookies bake up tender.

Add egg and vanilla: Mix in the egg and vanilla until fully combined and smooth.

Combine dry ingredients: In a separate bowl, whisk flour, cream of tartar, baking soda, and salt. Add to the wet ingredients and mix just until a dough forms. If crumbly, add 1 teaspoon milk at a time.

Chill the dough: Cover and refrigerate for 20–30 minutes. Chilled dough is easier to handle and spreads less in the air fryer.

Make cinnamon sugar: Stir 1/4 cup sugar with 1 1/2 teaspoons cinnamon in a shallow bowl.

Preheat the air fryer: If your model has a preheat function, set it to 320°F (160°C) for 3–4 minutes. If not, run it empty for a couple of minutes.

Form the cookies: Scoop 1–1.5 tablespoon portions, roll into balls, and coat thoroughly in cinnamon sugar.

Prepare the basket: Line the air fryer basket with a perforated parchment liner or lightly mist with nonstick spray. Leave room for air flow.

Arrange and flatten: Place 4–6 dough balls in the basket, spaced at least 1 1/2 inches apart. Gently press each ball to about 1/2-inch thick so they bake evenly.

Air fry: Cook at 320°F (160°C) for 6–8 minutes. Look for puffed centers with lightly set edges. Do not overbake; they’ll continue to set as they cool.

Cool in basket: Let cookies rest in the basket for 3–4 minutes, then transfer to a rack. This prevents crumbling and helps set the texture.

Repeat: Continue with remaining dough. If your basket runs hot, drop the time by 30 seconds for later batches.