Preheat your oven to 350°F (175°C) and line an 8-inch square baking pan with aluminum foil, allowing for overhang.

Melt the unsalted butter in a microwave-safe bowl in 20-second intervals until completely melted.

Stir in all-purpose flour, oats, granulated sugar, light brown sugar, and salt into the melted butter, reserving one cup of the mixture for topping.

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

In a separate bowl, combine mango, sugar, lemon juice, and corn starch, folding until mango is coated.

Spread the mango mixture evenly over the crust.

Sprinkle the reserved crumble mixture over the mango filling.

Bake in the preheated oven for about 50 minutes or until the top is golden brown.

Cool at room temperature for at least one hour before slicing into squares.