01 - Preheat your oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, ensuring there's overhang on the sides for easy removal.
02 - In a medium bowl, combine melted butter, granulated sugar, vanilla extract, all-purpose flour, and salt. Mix until a soft dough forms. Press this dough evenly into the bottom of the prepared baking pan. Bake for 15 minutes.
03 - While the crust is baking, toss the thinly sliced apples with granulated sugar, cinnamon, nutmeg, and all-purpose flour in a separate bowl until evenly coated.
04 - In another bowl, combine all-purpose flour, light brown sugar, salt, and cubed cold butter. Use your fingertips or a pastry blender to incorporate the butter until coarse crumbs develop.
05 - Spread the prepared apple filling evenly over the partially baked shortbread crust. Sprinkle the streusel topping uniformly over the apples. Bake for 35–40 minutes, or until the streusel is golden brown and the apples are tender.
06 - Allow the baked bars to cool completely in the pan before proceeding.
07 - In a small saucepan over medium heat, combine granulated sugar and water. Cook, swirling the pan gently, until the sugar dissolves and achieves a deep amber color, approximately 5–7 minutes. Remove the saucepan from the heat and carefully whisk in the cubed butter, followed by the heavy cream. Stir in the sea salt. Let the caramel cool slightly.
08 - Drizzle the slightly cooled salted caramel over the cooled bars. Remove the bars from the pan, slice them into 16 portions, and serve.