Prep Pan & Par-Bake Crust: Preheat oven to 300°F (150°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang handles. In a medium bowl, stir together melted butter, granulated sugar, vanilla, and salt. Stir in flour until a soft dough forms. Press evenly into bottom of pan. Bake for 15 minutes until set.

Prep Apple Filling: Increase oven temperature to 350°F (175°C). In a large bowl, toss finely diced apples with 2 tbsp flour, 2 tbsp sugar, cinnamon, and nutmeg until evenly coated. Spread apples evenly over warm shortbread crust.

Make Streusel Topping: In a medium bowl, whisk rolled oats, brown sugar, flour, and cinnamon. Cut cold cubed butter into mixture using a pastry blender or fingers until crumbly pea-sized clusters form.

Assemble & Bake: Scatter oat streusel evenly over the apple layer. Bake at 350°F for 30 to 35 minutes, or until the streusel is golden brown and apples are tender when pierced.

Caramel Drizzle & Chill: Remove from oven and drizzle warm salted caramel across the top. Cool completely at room temperature for 1 hour, then refrigerate for 1 hour for clean, bakery-neat square cuts. Slice into 16 squares and enjoy!