Preheat oven to 350°F (175°C) and line a 9x13-inch pan with parchment paper.
In a small skillet, melt butter and cook diced apples with cinnamon for 3-5 minutes until softened. Set aside to cool.
In a bowl, whisk flour, cream of tartar, baking soda, salt, and cinnamon.
In another bowl, beat butter, brown sugar, and granulated sugar until light and fluffy.
Add eggs and vanilla, mixing well.
Stir in dry ingredients until combined, then fold in cooled apples.
Spread dough evenly into the prepared pan and sprinkle cinnamon sugar on top.
Bake 25–30 minutes until edges are set and the center is slightly soft.
Cool completely before slicing into bars.