Prep your basics. Line two baking sheets with parchment. Set butter out until slightly soft to the touch. If your kitchen is hot, chill your bowl for a few minutes so the dough doesn’t get greasy.

Make the cinnamon-sugar. In a small bowl, mix 1/3 cup granulated sugar with 1–1.5 teaspoons cinnamon. Set aside for rolling.

Cream the butter and sugars. In a large bowl, beat 1/2 cup unsalted butter with 1/2 cup granulated sugar and 1/3 cup brown sugar until light and fluffy, about 2–3 minutes. This traps air for a softer cookie.

Add egg, vanilla, and starter. Beat in 1 large egg and 1 teaspoon vanilla until smooth. Mix in 1/2 cup sourdough starter (discard or active). The batter may look slightly curdled—that’s okay.

Whisk dry ingredients. In a separate bowl, whisk 1 3/4 cups all-purpose flour, 1 1/2 teaspoons cream of tartar, 1/2 teaspoon baking soda, 1/2 teaspoon fine sea salt, and a small pinch of cinnamon if using.

Combine gently. Add dry ingredients to the wet mixture. Stir with a spatula just until no dry streaks remain. Do not overmix or the cookies can turn tough.

Chill the dough. Cover and refrigerate 30–45 minutes. This helps the dough firm up, intensifies flavor, and controls spread.

Preheat and portion. Heat oven to 350°F (175°C). Scoop dough into 1.5-tablespoon balls (about 30 g each). Roll each ball in the cinnamon-sugar mixture until fully coated.

Bake. Place on prepared sheets, spacing 2 inches apart. Bake 9–11 minutes, until edges are set and centers still look slightly soft. The cookies will puff, then collapse slightly as they cool, creating that signature crinkle.

Cool and set. Let cookies rest on the sheet for 5 minutes, then transfer to a rack to finish cooling. They’ll firm up as they cool, staying chewy in the middle.