Prep oven and pan: Preheat oven to 350°F (175°C). Line a 9x9-inch metal square baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Make and blind-bake shortbread crust: In a food processor or bowl with a pastry cutter, pulse together flour, powdered sugar, cold butter cubes, vanilla, and salt until a crumbly dough forms. Press dough firmly and evenly into the bottom of the prepared pan. Bake for 15 minutes until lightly golden around the edges.
Prepare spiced apple filling: While crust bakes, toss diced apples with flour, granulated sugar, cinnamon, and nutmeg in a large bowl until evenly coated.
Make oat streusel: In a separate medium bowl, mix rolled oats, flour, brown sugar, and cinnamon. Cut in cold butter cubes using your fingers or pastry cutter until coarse, pea-sized crumbs form.
Assemble the layers: Spread the spiced diced apples evenly over the warm pre-baked shortbread crust. Scatter the buttery oat streusel evenly over the apples, gently pressing down.
Bake to bubbling perfection: Bake at 350°F for 30 to 35 minutes until the streusel is golden brown and the spiced apple juices are bubbling around the edges.
Cool, drizzle, and slice: Remove from oven and let cool completely in the pan for at least 1 hour (chill in the fridge for 30 minutes for cleanest square slices). Drizzle generously with warm salted caramel sauce, sprinkle with flaky sea salt, slice into 16 bars, and enjoy!