Place flour, baking powder, cornstarch and salt into a small bowl. Stir it with a whisker until all incorporated. Set aside.
Beat softened butter and brown sugar on high for 5 minutes, scraping the mixer bowl in between a couple of time. Add vanilla and mix for another minute or until all incorporated.
Add flour mixture and stir it in with on low for 3 minutes or until is all combined and the dough is formed.
Remove the dough from a mixer bowl into a plate, form a dough ball. Cut ¼ off from the dough ball. Wrap it into a food plastic wrap and place the small dough piece into a freezer.
Into a small bowl add flour, cinnamon, flour, salt and stir it all together with a hand whisker. Set aside.
Wash and peel apples. Chop apples into small pieces (I used onion chopper). Add flour mixture to apples, and mix it well. Add lemon juice and stir it once more until all incorporated.
Pinch off small pieces from the rest of the dough and drop them onto baking pan greased with butter and lined with parchment paper. Push the dough into the baking pan with your hand to form even dough layer. Add apple mixture and spread it out evenly.
Shred the frozen piece of dough on top of apples. Spread it out evenly with a fork. Bake in preheated oven for 45 min at 350 F. Remove from the oven let it cool in the pan.
Remove from the pan cut it into 12 even bars. Sprinkle with confectioner sugar or serve with ice cream. Enjoy!