Preparation: Preheat your oven to 350ºF (175ºC). Prepare a 9x9-inch baking dish by greasing it and lining it with parchment paper, leaving some overhang for easy removal later.

Create Cinnamon Sugar: In a small bowl, mix together the ground cinnamon and granulated sugar. Set aside.

Make the Dough: In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Add the egg and vanilla extract, mixing until well combined.

Combine Dry Ingredients: In a separate bowl, whisk together the flour, baking soda, cream of tartar, and kosher salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined. Avoid overmixing; the dough should be crumbly yet cohesive.

Assemble the Bars: Press half of the dough into the prepared baking dish in an even layer. Pour the thick caramel sauce over the dough and spread it gently, leaving a slight border around the edges. Crumble the remaining dough on top of the caramel layer. It's fine if some caramel peeks through. Sprinkle the cinnamon sugar evenly over the top.

Bake: Place the dish in the preheated oven and bake for 30-35 minutes or until the edges are golden brown and the center is set.

Finish: Once baked, remove the bars from the oven and immediately sprinkle flakey sea salt over the top for that perfect salty-sweet balance.

Cool and Serve: Allow the bars to cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang. Cut into squares or bars.