Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper or lightly grease it.
Prepare the crust and topping:
In a bowl, combine flour, sugar, baking powder, and salt.
Cut in cold butter using a pastry cutter or fork until the mixture resembles coarse crumbs.
Stir in the beaten egg to form a dough.
Press about ⅔ of the dough into the bottom of the prepared baking pan to form the crust.
Make the cherry filling:
In a separate bowl, mix cherry pie filling and cornstarch (if using). Stir to combine and thicken the filling.
Spread the cherry filling evenly over the crust.
Add the crumble topping:
Crumble the remaining dough over the cherry filling, covering as much of the surface as possible.
Bake for 40–45 minutes, or until the crust and topping are golden brown.
Let the bars cool completely in the pan on a wire rack. Slice into squares and serve.