Pre-heat oven to 350 degrees F.
Cut mangoes into small chunks. In a bowl, combine mango chunks, sugar (use more if mango is not ripened) and corn starch. Mix well and set aside.
In a large bowl, combine flour, baking powder and salt.
Cut cold (important: make sure it is cold!) unsalted butter into chunks and add to flour mixture.
If using a food processor, pulse until small crumbs are formed. If using a pastry cutter, cut butter into flour mixture until small crumbs are formed.
Mix together wet ingredients - brown sugar, egg and vanilla extract. Then add wet ingredients into buttery dry mixture. Mix well - mixture should be slightly crumbly.
Bring ~2/3 of crumbly mixture to the bottom of a 8" x 8" baking pan lined with parchment paper. Press mixture onto pan to form a layer of crumb pastry.
Add mango mixture, and spread it out evenly.
Then sprinkle the top with the rest of the crumbles.
Bake in the oven for 35-40 minutes.
Remove from oven and let it cool for 5-10 minutes. Then lift parchment paper and bring pastry out from the pan. Let it cool for another 15-20 minutes.
Cut into 9 square bars. Serve and enjoy!