Preheat oven to 350 degrees F and line an 8x8 baking pan with parchment paper, set aside.
In the bowl of a stand mixer add flour, butter, and powdered sugar.
Mix on medium speed until everything has combined and forms a soft dough. The mixture may look very crumbly at first but will form together as a dough.
Evenly press the dough into the prepared pan.
Bake for 20 minutes or until the edges just start to brown.
While the crust is baking start the filling.
For filling:
In a medium-sized mixing bowl add sliced apples, flour, granulated sugar, and apple pie spice.
Toss to combine.
Add the sliced apples overtop the crust.
For topping:
In a medium-sized mixing bowl add all the streusel ingredients and mix until combined and crumbly.
Evenly sprinkle the streusel overtop the apples.
Add to the oven and bake for 35-40 minutes or until the streusel topping is golden brown.
Remove from the oven and allow to cool.
Once cool, cover and add to the fridge for at least 2 hours but overnight is best.
Cut into squares and serve with ice cream or whipped cream.