Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
In a medium bowl, whisk together flour, cream of tartar, baking soda, salt, and cinnamon.
In a large bowl, beat softened butter and sugar until light and fluffy. Add eggs and vanilla; mix well.
Gradually add dry ingredients to the wet mixture until combined.
Fold in finely diced apples.
In a small bowl, mix ¼ cup sugar and 2 tsp cinnamon for coating.
Roll dough into balls and coat each in the cinnamon sugar mixture.
Place on baking sheet and bake for 10–12 minutes, until edges are set but centers remain soft.
Let cookies rest for 2 minutes on the pan, then transfer to a wire rack to cool.