In a bowl, combine the thinly apples, lemon juice, granulated sugar, ground cinnamon, and a pinch of salt. Toss until the apples are evenly coated.
Place the apple mixture in a medium to a large pot over medium-high heat. Slowly bring it to a boil, stirring occasionally.
In a separate bowl, create a slurry by mixing cornstarch with cold water. Set it aside.
As the apples release liquid and begin to soften, add the slurry to the mixture while stirring constantly. The mixture should thicken within about 30 seconds. If needed, prepare a bit more slurry to reach the desired thickness.
Once thickened, remove the apple mixture from heat and transfer it to a container. Let it cool down to room temperature . The filling can be stored in an airtight container in the fridge for up to 1 week.
For the Cinnamon Streusel:
In a mixing bowl, whisk together the brown sugar, salt, cinnamon, and flour.
Add the cold cubed unsalted butter and combine everything with your hands until it has a shaggy texture. This takes a minutes, so be sure to really work it! Store it in an airtight container in the fridge for up to 7 days if prepared in advance.
For the Cookie Dough:
Preheat the oven to 350 degrees. Line an 8x8 or 9x9 pan with parchment paper and spray it with cooking spray. Set it aside.
In a microwave-safe bowl, melt the butter.
In a medium-sized bowl, whisk the melted butter and the sugars until the mixture is light and fluffy, about 3 minutes.
Whisk in the egg and egg yolk for 30 seconds and then add the vanilla. Whisk just until combined. Add the flour, baking powder, corn starch salt, and cinnamon.
Switch to a rubber spatula and fold the dry ingredients into the wet ingredients until there are no more signs of flour. Do not overmix.
Press the dough into the prepared pan. Add the apple filling on top of the dough and, with the back of a spoon, evenly spread it across the top of the dough. Sprinkle the streusel on top.
Bake for 30-35 minutes until the top is golden brown.
Let it cool completely before cutting. Serve at room temperature or warm with ice cream.