1-First, preheat your oven to 300°F (150°C) and line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal. This helps the crust bake evenly and prevents sticking.

2-Next, mix the melted butter, granulated sugar, vanilla extract, and salt in a bowl, then add the flour to form the shortbread crust. Press this mixture evenly into the prepared pan and bake for 15 minutes. While it bakes, toss the apple slices with flour, granulated sugar, cinnamon, and nutmeg to create the filling.

3-After removing the crust, increase the oven to 350°F (175°C). Layer the spiced apples over the warm crust, pressing them down gently. For the streusel, combine oats, brown sugar, cinnamon, flour, and cold cubed butter until it forms coarse crumbs, then sprinkle it over the apples.

4-Bake for 30 to 35 minutes until the topping turns golden brown. Let the bars cool at room temperature for 20 minutes, then chill in the refrigerator for at least 2 hours. Once set, cut into bars and drizzle with salted caramel sauce. With about 25 minutes of prep, 45 minutes of baking, and 2 hours chilling, you’re looking at around 3 hours and 20 minutes total.