Creating perfect Mango Oatmeal Crumble Bars is simple when you follow these steps:

1. Preheat the Oven: Begin by preheating your oven to 350°F (175°C). Grease a 9×9-inch baking dish with non-stick spray or line it with parchment paper.

1. Prepare the Mango: Peel and dice the mangoes. In a medium bowl, toss the diced mango with lemon juice and set aside.

1. Mix Dry Ingredients: In a large bowl, combine the rolled oats, all-purpose flour, brown sugar, granulated sugar, baking powder, salt, and ground cinnamon if using.

1. Add Wet Ingredients: Pour the melted butter and vanilla extract into the dry mixture. Stir until well combined and crumbly.

1. Layer the Mixture: Press half of the oat mixture evenly into the bottom of the prepared baking dish, creating a solid base.

1. Add Mango Layer: Spread the diced mango over the base layer evenly, making sure each piece is well distributed.

1. Top with Oat Mixture: Sprinkle the remaining oat crumble mixture over the mango layer, ensuring even coverage.

1. Bake: Place the baking dish in the preheated oven and bake for about 30 minutes, or until the top is golden brown and the edges are bubbling slightly.

1. Cool: Remove the dish from the oven and allow it to cool for around 15 minutes in the pan. This will help firm up the bars for easier cutting.

1. Slice & Serve: Once cooled, cut into squares or bars, and serve as desired.