Begin by preheating your oven to 350°F (175°C). Prepare a 9x13-inch baking pan by lining it with parchment paper, ensuring there is an overhang for easy lifting later.
In a large mixing bowl, use an electric mixer to cream together the softened butter, packed brown sugar, and granulated sugar. Beat on medium speed for about 2-3 minutes, until the mixture is light, fluffy, and well-combined.
Add the eggs to the sugar mixture one at a time, beating well after each addition. Stir in the vanilla extract until everything is nicely incorporated.
In a separate bowl, whisk together the all-purpose flour, baking powder, and salt until evenly mixed. Gradually add this dry mixture to the wet ingredients, mixing on low speed just until everything is combined; do not overmix.
Set aside 1 cup of the prepared batter for later. Spread the remaining batter evenly into the bottom of the prepared baking pan, smoothing it out with a spatula.
Generously drizzle the caramel sauce over the batter, ensuring it is evenly distributed. Then, using a spoon, dollop the reserved 1 cup of batter over the caramel layer in small spoonfuls, allowing some of the caramel to show through.
Place the baking pan in the preheated oven and bake for approximately 25-30 minutes. The bars are done when the top is a light golden brown and a toothpick inserted into the center comes out with a few moist crumbs clinging to it.
As soon as you take the bars out of the oven, sprinkle the flaky sea salt evenly over the top, enhancing the flavor with a wonderful salty contrast.
Allow the bars to cool completely in the pan on a wire rack. Once cooled, lift them out using the parchment overhang and cut into squares.
Notes
Present the bars in a charming rustic wooden tray or plate. For an added touch of indulgence, drizzle additional caramel sauce over the top just before serving. Enjoy your delightful treat!