Begin by preheating your oven to 350°F (175°C). Prepare a 9x13 inch baking dish by greasing it liberally with butter, or line it with parchment paper for easy lifting after baking.
In a large mixing bowl, use a hand mixer or a stand mixer on medium speed to cream the softened butter with the granulated sugar and brown sugar. Mix until the mixture becomes light and fluffy, roughly 3-4 minutes.
Add the eggs to the butter-sugar mixture one at a time, ensuring each is well blended before adding the next. Follow this by mixing in the pure vanilla extract until fully incorporated.
In another mixing bowl, combine the flour, cream of tartar, baking soda, sea salt, ground cinnamon, and ground nutmeg. Whisk these dry ingredients together until evenly mixed.
Gradually incorporate the dry ingredients into the wet batter, mixing gently with a spatula or the mixer on low, just until no flour streaks remain. If opting for white chocolate chips, gently fold them into the batter now.
Pour the thick batter into the prepared baking dish and use a spatula to spread it evenly across the surface. Take care to smooth the top.
Evenly sprinkle the cinnamon sugar mixture over the entire surface of the batter, ensuring an irresistible sweet crust.
Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the edges turn golden brown and a toothpick inserted into the center comes out clean.
Once baked, let the bars cool in the pan for about 15 minutes. Afterward, gently lift them out using the parchment paper (if used) and transfer them to a wire rack to cool completely.
When fully cooled, slice your delightful apple snickerdoodle bliss bars into squares and indulge in this heavenly treat!
Notes
For a delightful touch, serve the bars on a rustic wooden platter with a sprinkle of cinnamon sugar on top and a few thinly sliced apples arranged alongside for a colorful presentation.