Preheat the oven to 350°F.

Line an 8-inch square baking pan with parchment paper. Set aside.

To make the crust, place the softened butter, brown sugar, vanilla, and salt in a medium bowl and use a fork to combine. Add the flour and cinnamon, and continue mixing until a soft dough forms. Transfer the dough to the baking pan.

Using a tamper, clean hands, or the bottom of a heavy glass, press the dough into an even layer.

Bake the crust on the middle rack of oven until slightly golden at the edges, 15-18 minutes. While the crust bakes, prepare the streusel.

Place the flour, oats, brown sugar, granulated sugar, spices, and salt in a medium bowl and whisk to combine. Pour the melted butter over the mixture and stir with a fork until large crumbs form. Transfer the bowl to the refrigerator to chill while making the filling.

Peel and core the apples. Cut the apples into 1/8-inch thick slices, and cut the slices in thirds crosswise.

Place the apples in a large mixing bowl. Add the lemon juice and vanilla and stir to combine. Sprinkle the flour, sugar, and spices over the apples, and toss until the apples are evenly coated.

Carefully remove the baking pan from the oven. Spoon the apples on top of the crust and arrange them into an even layer, gently pressing down. Sprinkle the apple layer evenly with the chilled streusel.

Return pan to oven and continue baking until streusel is golden brown and the apple juices bubble slightly at the edges of the pan, 30-35 minutes.

Check to see that bars are done. Remove from oven or add time as needed.

Allow the bars to cool for 20 minutes at room temperature, then transfer them to the refrigerator. Chill for at least 2 hours before slicing and serving.

Lift the bars from the pan by pulling up on the edges of the parchment paper. Slice into squares and serve.