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

In a large mixing bowl, melt ½ cup unsalted butter and mix with other crust ingredients until sandy.

Set aside 1 heaping cup of the oat mixture for topping. Press the rest into the bottom of the pan to form the crust.

In the same bowl, combine mango, granulated sugar, lemon juice, and corn starch; toss together until evenly coated.

Spread the mango mixture evenly over the crust and sprinkle with reserved crumble topping.

Bake for 50 to 55 minutes until edges are set and center is bubbling; cool at room temperature for at least 1 hour.

Lift the bars from the pan using the foil overhang and slice into squares.