Preheat the oven to 350℉.
For the blueberry filling, add the blueberries to a pot with the lemon juice and sugar. Heat on medium until everything begins to bubble.
Add the cornstarch and water to the mixture and stir together while it continues to boil.
Let the blueberry filling simmer on low heat for about 5 minutes, stirring every other minute. Remove from the heat.
In a bowl add softened butter and whisk together until it becomes a lighter shade. Add the brown and white sugar to the butter and mix until fluffy. Add the lemon juice and vanilla extract, whisking together until combined.
Next add the salt and the flour to the bowl and mix until well incorporated. You should have a thick dough.
Add parchment paper to a 9x9 inch square baking pan. Take ¾ of the dough and press into the bottom of pan, spreading evenly.
Then add a layer of the blueberry filling, spreading to cover the dough layer.
With the rest of the dough, crumble it on top of the blueberry filling.
Bake in the oven for 40-45 minutes.
Let cool completely before cutting and sifting powdered sugar on top if desired. Enjoy!