Pre-heat oven to 350°F/180°C. Line a 9×9-inch (23cm) pan with parchment paper.
In a large mixing bowl, whisk together the flour, oats, both sugars, salt and baking powder together well. Using a pastry blender or your fingertips, cut or rub in the butter until large crumbs form. Set aside 1 cup of the crumb mixture (to use as the topping).
Press the remaining crumb mixture firmly and evenly into the bottom of the baking pan and set aside.
In a medium mixing bowl, mash the mangoes, sugar, lemon juice and cornstarch together. You can mash or blend it to a puree, or leave some chunks for texture.
Spread the mango mixture evenly over the crust in the baking tray, then sprinkle the remaining crumb mixture loosely over the top.
Bake for 45 minutes or until the crumb topping is golden and the mango filling is bubbling.
Allow the mango bars to cool in the pan, then remove from the pan and cut into bars.
Notes
When choosing a ripe mango, make sure to give it a gentle squeeze. If it's too hard, it's not ripe yet. If it yields to the touch, it's overripe. You want a mango that is just ripe - soft but with a little give.If you don't have fresh mangoes on hand, you can use frozen mangoes instead! Just make sure to thaw them before using them.If you want a mango flavor without the chunks, you can use mango puree instead of fresh mangoes.After making the crust, store it in the fridge while you assemble the filling to ensure the butter stays cold and firm.Do not use quick-cooking oats for this recipe as they will turn to mush when baked.