Prep the apple: Peel and core 1 small apple. Finely dice into tiny pea-size pieces. Toss with a squeeze of lemon juice if you like. Pat dry with a paper towel to remove excess moisture.

Make cinnamon-sugar: In a small bowl, combine 1/3 cup granulated sugar with 1 1/2 teaspoons cinnamon. Set aside for rolling.

Whisk dry ingredients: In a medium bowl, whisk 2 3/4 cups all-purpose flour, 1 teaspoon cream of tartar, 1/2 teaspoon baking soda, 3/4 teaspoon fine sea salt, 1 teaspoon cinnamon, and a pinch of nutmeg.

Mix wet ingredients: In a large bowl, whisk 3/4 cup melted unsalted butter (slightly cooled), 3/4 cup granulated sugar, and 1/2 cup packed light brown sugar until smooth. Add 1 large egg and 2 teaspoons vanilla; whisk until glossy.

Combine: Add dry ingredients to the wet and stir with a spatula until a soft dough forms. Fold in the diced apple. The dough should be thick and slightly tacky, not sticky.

Chill: Cover and refrigerate the dough for 30–45 minutes. This helps the cookies hold their shape and keeps the apple bits from releasing too much moisture.

Preheat and prep: Heat the oven to 350°F (175°C). Line two baking sheets with parchment paper.

Shape and coat: Scoop the dough into 1 1/2-tablespoon balls. Roll each ball in the cinnamon-sugar until well coated. For extra sparkle, roll twice.

Bake: Arrange 2 inches apart on the sheets. Bake 9–11 minutes, until edges look set and centers are puffed but soft. Do not overbake; they’ll finish on the sheet.

Cool: Let cookies rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. The centers will settle into a soft, chewy texture.