Add diced mangoes, granulated sugar, unsalted butter, and cornstarch to a non-stick pan and mix well.
Cook for 10 minutes on medium heat until bubbly and thickened. Stir frequently while cooking.
Combine all-purpose flour, old-fashioned oats (rolled oats), baking powder, granulated sugar, and ground cardamom in a medium-size mixing bowl.
Add cold unsalted butter (cut into 1-inch cubes) to the bowl and mix using your fingertips to make a crumbly mixture. Do not over mix. You can also use a fork to mix the butter with the flour mixture.
Preheat the oven to 375 degrees F (190 degrees C). Transfer the mango filling to an 8×8 baking pan and spread evenly
Spread the flour mixture evenly over the mango mixture.
Keep the pan in the middle rack of the oven and bake for 35-40 minutes until the edges are bubbling and the top is browned.