Preheat Oven: Set the oven to 350°F. Grease a 9x9-inch pan and line with parchment paper, leaving extra paper hanging over the edges for easy removal.
Prepare Dry Ingredients: In a medium bowl, mix together the flour, cream of tartar, baking soda, and salt. Set aside.
Beat Butter and Sugars: In a large bowl or stand mixer, beat the softened butter for 2 minutes. Add the granulated and brown sugar, and beat for another 2 minutes until well combined.
Add Eggs and Vanilla: Scrape down the bowl, then beat in the eggs one at a time, making sure each egg is fully mixed in before adding the next. Add the vanilla extract and mix well.
Combine Ingredients: Add the dry ingredients to the butter mixture. Mix on low speed until almost combined, then finish mixing with a spatula to ensure all the flour is incorporated.
Layer Dough: Spread half of the dough evenly in the prepared pan. Sprinkle 2 tablespoons of the cinnamon sugar on top.
Add Dulce de Leche: Spread the dulce de leche evenly over the dough layer.
Top and Sprinkle: Spread the remaining dough over the dulce de leche. Sprinkle the rest of the cinnamon sugar on top.
Bake: Bake for 25-30 minutes, until the edges are set and a toothpick inserted in the center comes out clean.
Cool and Serve: Let the bars cool in the pan for 15 minutes, then use the parchment paper to lift them out. Cool for another 10 minutes before slicing and serving.