Preheat your oven to 350°F (175°C) and line an 8-inch square baking pan with foil, leaving some overhang, and lightly coat it with cooking spray.

In a microwave-safe bowl, melt the unsalted butter on high for about 30-45 seconds until smooth. Mix in all-purpose flour, oats, granulated sugar, brown sugar, and salt until crumbly. Set aside a cup for topping.

Press the remaining mixture into the bottom of the prepared pan evenly to create the crust.

In a separate bowl, toss chopped mango with sugar, lemon juice, and corn starch until evenly coated. Spread this mixture over the crust.

Sprinkle the reserved crumble mixture over the mango layer evenly.

Bake for 50 minutes, until edges are light golden and filling is bubbly.

Cool for at least 1 hour at room temperature before lifting out of the pan and slicing into squares.