Preheat the oven to 350F. Line an 8X8 square pan with parchment paper, leaving overhang on the sides so you can lift them up when they are finished.

In a large bowl. add the melted butter and sugar. Mix with a non-stick spatula until combined. Add in vanilla and mix again.

Pour flour into the bowl, adding the salt directly on top of the flour and mix until very thick dough forms.

Press dough (you can use your hands or the back of a measuring cup!) into the prepared pan in an even layer, making sure to smooth out the top.

Bake for at least 25 minutes until the edges are lightly browned and the center is set. Remove from oven and set aside.

In a large saucepan over medium-low heat, add the butter. Let the butter melt and add in the apples. Sauté until slightly softened, about 8 minutes. Add in the brown sugar and cinnamon and continue to cook, stirring often, until apples are soft and caramelized, stirring often, about 5 more minutes. Stir well and add in the flour. Mix flour thoroughly into the liquid in the pan ensuring that there are no clumps. Bring heat up to medium- medium-high and bring the apples to a simmer, let simmer for 2 minutes and turn off the heat. At this point, the apples should be soft and the liquid surrounding them should have thickened. Set aside.

Combine all ingredients in a large bowl, mixing with a non-stick spatula or wooden spoon until a clumpy thick dough forms.

Pour the apple filling over the top of the par-baked shortbread crust. Smooth the top into an even layer with a spatula.

Crumble the streusel on top of the apple filling making sure to cover the entire pan.

Bake at 350 for at least 35 minutes, or until the top is golden brown. This could take up to 50 minutes depending on your oven. Remove from oven and let cool in the pan. If topping with salted caramel, drizzle on top when bars are cooled. Slice and enjoy!