Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish or line it with parchment paper for easy removal.
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Incorporate the vanilla extract.
In a separate bowl, whisk together the flour, cream of tartar, baking soda, salt, cinnamon, and nutmeg.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. If you're using white chocolate chips, fold them into the batter at this stage.
Spread the batter evenly into the prepared baking dish. Smooth the top with a spatula.
Sprinkle the cinnamon sugar mixture evenly over the top of the batter.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden and a toothpick inserted in the center comes out clean.
Allow the bars to cool in the pan for about 15 minutes before transferring them to a wire rack to cool completely.
Once cooled, cut into squares and enjoy your delicious apple snickerdoodle bars!
Notes
For a richer flavor, consider adding white chocolate chips.