Preheat the oven to 375°F (190°C). Lightly grease an 8- or 9-inch baking dish.
In a bowl, whisk flour, brown sugar, baking powder, salt, and cinnamon. Add melted butter and mix until coarse crumbs form.
In a large bowl, toss apples with sugar, lemon juice, cornstarch, salt, and cinnamon until evenly coated.
Spread the apple mixture evenly into the prepared dish.
Sprinkle the crumble topping evenly over the apples.
Bake the Classic Apple Crumble for 35–45 minutes until golden and bubbly.
Cool for 15 minutes before serving.