Pre-heat oven to 350 degrees F.
In a bowl, combine blueberries, sugar, corn starch, and lemon juice (optional). Mix well and set aside.
In a large bowl, combine flour, baking powder and salt.
Slice cold unsalted butter (important: make sure it is cold!) into multiple chunks and add to flour mixture.
Use a pastry cutter to cut butter into dry mixture until smaller crumbs are formed. Or, using a food processor, pulse until small crumbs are formed.
Mix together wet ingredients in a separate bowl – brown sugar, egg and vanilla extract. Then add wet ingredients into buttery dry crumble mixture. Mix well – mixture should be slightly crumbly.
Bring ~3/4 of crumbly mixture to the bottom of a 8″ x 8″ baking pan lined with parchment paper. Press mixture using spatula onto pan to form a layer of crumb pastry.
Add blueberry mixture onto the layer of crumb pastry, and spread it out evenly.
Sprinkle the top of the blueberry mixture 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 25-30 minutes.
Cut into 9 square bars. Serve and enjoy!