Preheat your oven to 350°F (175°C) and prepare an 8-inch square baking pan with aluminum foil and greasing.

Melt unsalted butter in a microwave-safe bowl. Stir in flour, oats, granulated sugar, brown sugar, and salt. Reserve 1 cup for topping.

Press the remaining mixture into the bottom of the prepared pan, creating a compact base.

In a separate bowl, combine chopped mango, sugar, lemon juice, and corn starch, mixing gently.

Pour the mango mixture evenly over the crust and spread it gently.

Sprinkle the reserved crumble mixture evenly over the mango layer.

Bake for 50 minutes until edges are golden brown and the center is firm.

Let cool in the pan for 1 hour before lifting out and slicing into squares.