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

Melt ½ cup of unsalted butter in a microwave-safe bowl. Add flour, oats, granulated sugar, light brown sugar, and salt. Mix until well combined.

Set aside one cup of the crumble mixture for topping. Press the remaining mixture into the bottom of the prepared pan.

In a separate bowl, mix chopped mango, additional sugar if desired, lemon juice, and corn starch until well coated.

Spread the mango mixture evenly over the crust, then sprinkle the reserved crumble on top.

Bake for approximately 50 minutes until edges are golden and center is bubbling.

Cool in the pan on a wire rack for at least 1 hour before lifting out and slicing into squares.