Preheat your oven to 350°F (175°C) and line an 8-inch square pan with aluminum foil, lightly coating it with cooking spray.

Melt the unsalted butter in a large microwave-safe bowl, then mix in all-purpose flour, oats, granulated sugar, light brown sugar, and salt until the mixture resembles a dry, sandy texture.

Set aside a heaping cup of the crumble mixture for topping later.

Press the remaining mixture firmly into the bottom of the prepared pan to form a crust.

In another bowl, combine the chopped mango, additional sugar, lemon juice, and corn starch; stir until well combined.

Spread the mango mixture evenly over the crust.

Sprinkle the reserved crumble mixture over the mango layer.

Bake for about 50 minutes or until bubbling edges and a set center are observed.

Allow the bars to cool in the pan for at least 1 hour before lifting them out and slicing into squares.