Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish or line it with parchment paper, leaving some overhang for easy removal later.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until the mixture is light and fluffy (about 3-4 minutes).
Beat in the eggs one at a time, followed by the vanilla extract. Mix until fully incorporated.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, mixing just until combined.
In a small bowl, toss the diced apples with cinnamon to coat them evenly.
Gently fold the cinnamon-coated apples into the batter, being careful not to overmix.
Pour the apple batter into the prepared baking dish, spreading it evenly.
Drizzle the caramel sauce over the top of the batter, then use a knife or a skewer to swirl it slightly for a marbled effect.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Let the bars cool in the pan for about 10 minutes, then lift them out using the parchment overhang. Transfer to a wire rack to cool completely before slicing into bars.
Once cut, sprinkle sea salt generously over the top of the bars for that perfect salted caramel balance.
Notes
For best results, use a mix of sweet and tart apples.