Start by mixing melted butter, sugar, vanilla, and salt until smooth. Stir in the flour just until combined — don’t overmix. Press the mixture evenly into a parchment-lined 8×8-inch pan. Bake at 300°F (150°C) for about 15 minutes until slightly golden and set.
While the crust bakes, combine sliced apples with flour, sugar, cinnamon, and nutmeg until evenly coated. In a separate bowl, mix oats, brown sugar, cinnamon, and flour, then cut in the cold butter until the mixture forms coarse crumbs. The butter must stay cold for that perfect crumbly texture.
Once the crust is out of the oven, increase the temperature to 350°F (177°C). Spread the apple mixture evenly over the warm crust. Then sprinkle the streusel evenly on top. Bake for 30–35 minutes, until the top is golden brown and the apples are tender.
Let the bars cool at room temperature for about 20 minutes, then refrigerate for at least 2 hours to set. Once chilled, lift them out of the pan using the parchment overhang, cut into squares, and drizzle generously with salted caramel sauce before serving.