Preheat the oven to 350 degrees Fahrenheit.
Grease an 11x11" baking pan and set aside
In a large bowl mix together flour, oats, salt and baking powder with a wooden spoon until combined.
Add in coconut oil or butter, brown sugar and vanilla extract until the mixture is crumbly.
Press two-thirds of the crumble mixture into the prepared baking pan.
Reserve remaining crumble for later.
Spread chopped mangoes over crumble in pan, then drizzle lemon juice over top. Sprinkle remaining crumble on top of bars.
Bake for 25 to 30 minutes or until crumble is golden brown.