Preheat the Oven: Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.
Prepare the Crust: In a large mixing bowl, combine flour, sugar, salt, and cinnamon. Pour in the melted butter and mix until the mixture resembles coarse crumbs.
Set Aside Crust: Reserve 1 cup of the crust mixture for later use. Press the remaining mixture firmly into the bottom of the prepared pan.
Prepare the Filling: In another bowl, toss the sliced apples with lemon juice, cornstarch, and vanilla extract until well coated.
Assemble Bars: Spread the apple mixture evenly over the crust layer. Sprinkle the reserved crust mixture evenly on top of the apples.
Bake: Bake in the preheated oven for about 30-35 minutes or until the topping is golden brown and the apples are tender.
Cool and Serve: Allow the bars to cool in the pan before slicing them into squares. Serve warm with a drizzle of caramel sauce or alongside a scoop of vanilla ice cream.