First Step: Preheat and Prepare Your Pan Begin by preheating your oven to 300°F (150°C). Line an 8-inch (20 cm) square baking pan with parchment paper, allowing some overhang for easy removal of the bars later.

Second Step: Make the Shortbread Crust In a medium bowl, combine melted butter, granulated sugar, vanilla extract, salt, and flour. Mix until the dough forms and press it evenly into the bottom of the prepared pan. Bake the crust for 15 minutes; this sets a sturdy but tender base.

Third Step: Prepare the Apple Filling While the crust bakes, peel and slice or chop the apples. Toss them with flour or cornstarch, sugar, cinnamon, nutmeg, and lemon juice if using. This mixture thickens during baking and infuses warm spice flavors.

Fourth Step: Make the Streusel Topping Combine oats, brown sugar, cinnamon, and flour in a bowl. Cut in cold cubed butter using a pastry cutter, forks, or your fingertips until coarse crumbs form. If mixture feels too dry, gently warm your hands to bind the butter workings better. For better texture, chill streusel topping if desired.

Fifth Step: Assemble the Bars Increase oven temperature to 350°F (177°C). Spread apple filling evenly over the warm crust, pressing gently to fit. Sprinkle the streusel topping evenly over the apples.

Sixth Step: Bake the Bars Bake for 30-35 minutes or until the streusel topping is golden brown and crunchy. The filling should be bubbling and cooked through for perfect consistency.

Final Step: Cool, Chill, and Serve Allow the bars to cool at room temperature for at least 20 minutes. Transfer to the refrigerator and chill for a minimum of 2 hours or overnight to set fully. Cut into 12-16 squares and serve. For an extra touch, drizzle with salted caramel sauce before serving.