Start by preheating your oven to 375°F (190°C) and lightly greasing a 9x13-inch baking dish.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt until well combined. Then, mix in the cubes of cold butter using your fingers or a pastry cutter until you achieve a sandy texture.
Whisk the egg with milk and stir this mixture into the flour blend until it forms a soft, cohesive dough.
Press approximately half of the dough into the bottom of the prepared baking dish, forming an even layer.
For the cherry filling, mix the cherries with cornstarch, vanilla extract, and a couple of tablespoons of sugar in a separate bowl. Spread this cherry mixture evenly over the crust.
Crumble the remaining dough on top of the cherries, creating a rough and rustic look.
Bake in the preheated oven for about 30 to 35 minutes, or until the topping is golden and the cherries are bubbling at the edges.
Allow the bars to cool completely in the dish before slicing them into squares. This helps them hold their shape.