Preheat oven to 300°F (150°C). Line the bottom of an 8x8-inch baking pan with parchment paper, leaving edges over the sides for easy removal. Alternatively, use a high-quality nonstick aluminum foil.

In a medium bowl, combine melted butter, granulated sugar, vanilla extract, and salt. Stir until combined.

Fold in the flour until the mixture is combined.

Press the crust mixture evenly into the bottom of the prepared baking pan. Bake for 15 minutes at 300°F (150°C).

While the crust bakes, prepare the apple filling. In a large bowl, combine sliced apples, flour, granulated sugar, cinnamon, optional ginger, and nutmeg. Stir until the apples are evenly coated.

Prepare the crumble topping. In a medium bowl, combine flour, brown sugar, and softened butter. Use a fork to cut the butter into the dry ingredients until a crumbly texture is achieved.

Increase oven temperature to 350°F (175°C).

Evenly spread the apple filling mixture over the warm crust using a rubber spatula.

Evenly sprinkle the crumble topping mixture over the apple filling. Bake for 30 to 35 minutes at 350°F (175°C), or until the crumble is golden brown and the apple filling is bubbling.

Remove the pan from the oven and let the bars cool on a wire rack for at least 2 hours, or overnight. Do not cover, as this will soften the topping.

Once cooled, drizzle generously with salted caramel sauce. Slice and serve.