Prepare Apple Mixture: Dice 250g Gala apples finely (1/4 inch). Sauté apples in 1 tbsp unsalted butter. Add 50g dark brown sugar, 0.25 tsp ground cinnamon, 1 small lemon juice, and 1 tbsp water. Cook for 5-7 minutes until tender and slightly softened. Cool completely before use.
Whisk Dry Ingredients: In a medium bowl, whisk 250g all-purpose flour, 1 tsp cream of tartar, 1 tsp baking soda, 0.5 tsp fine sea salt, 5g cornstarch, and 1.25 tsp ground cinnamon. Set aside.
Cream Wet Ingredients: In a stand mixer, cream 184g softened unsalted butter and 100g granulated sugar until light and fluffy (3-5 mins). Beat in 1 large egg until just combined, then mix in 2 tsp vanilla extract.
Combine Dough: Gradually add the whisked dry ingredients to the wet mixture on low speed, mixing only until flour streaks disappear. Do not overmix. Gently fold in the completely cooled apple mixture until evenly distributed.
Chill Dough and Prepare Coating: For best results, chill the cookie dough for at least 30 minutes. While dough chills, prepare the cinnamon-sugar coating: In a shallow bowl, combine 66g cinnamon sugar mixture (1/3 cup granulated sugar + 1 tbsp ground cinnamon).
Shape and Bake Cookies: Preheat oven to 375°F (190°C). Line two large baking sheets with parchment paper. Scoop 1.5-2 tbsp portions of chilled dough, roll into smooth balls, and generously coat each ball in the cinnamon-sugar mixture. Place on prepared baking sheets, 2 inches apart.
Cool and Serve: Bake one sheet at a time for 9-12 minutes, until edges are set and centers appear slightly underbaked. Do not overbake. Let cookies cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely. Repeat with remaining dough and enjoy.
Notes
Chilling the dough improves the texture of the cookies.
Make sure to not overbake the cookies to maintain a chewy center.