01 - Preheat oven to 350°F. Line a 9x9 inch baking pan with parchment paper, ensuring an overhang for removal.
02 - In a mixing bowl, whisk together flour, granulated sugar, and salt. Cut in the cold cubed butter using a pastry cutter or fork until the mixture forms coarse crumbs. Blend in vanilla extract. Press two-thirds of the mixture firmly and evenly into the base of the prepared pan. Freeze for 10 minutes.
03 - Melt butter in a skillet over medium heat. Add diced apples, brown sugar, granulated sugar, cinnamon, nutmeg, lemon juice, and a pinch of salt. Cook, stirring, until apples are tender yet hold their shape, about 5 to 7 minutes. Remove from heat and let cool slightly.
04 - Distribute the apple filling evenly over the chilled crust. Sprinkle the remaining crust mixture as a crumble topping.
05 - Bake for 30 to 35 minutes, or until the top is golden brown and set. Allow to cool completely in the pan on a wire rack.
06 - In a saucepan over medium heat, melt granulated sugar, gently swirling until liquefied and amber in color. Add cubed butter and whisk until fully incorporated. Remove from heat and gradually whisk in warm heavy cream until smooth. Stir in flaky sea salt. Cool slightly to achieve a pourable consistency.
07 - Using the parchment paper overhang, lift the cooled slab from the pan. Cut into 16 bars and drizzle with salted caramel just before serving.