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

Melt the unsalted butter in a microwave-safe bowl in 30-second intervals until fully liquid.

Combine melted butter with all-purpose flour, oats, granulated sugar, brown sugar, and salt until the mixture resembles wet sand.

Reserve one heaping cup of the mixture, then firmly press the remaining into the bottom of the prepared pan.

In a separate bowl, mix the chopped mango, sugar, lemon juice, and cornstarch until well coated.

Pour the mango mixture over the crust and sprinkle the reserved crumb topping evenly.

Bake for about 50 minutes, or until the edges are set and the top is golden.

Let cool for at least 1 hour, then lift the bars out using the foil and slice into squares.