Spray a 9 x 13 baking pan with cooking spray and preheat oven to 350 degrees.
In a medium bowl, combine the 2 cups flour, baking powder, and salt. Set aside.
In a large bowl, mash the bananas until liquefied.
Mix in the 1/2 cup butter and sugar until combined.
Stir in the eggs, vanilla, and milk.
Add in the flour mixture from step 1 as well as the milk, stirring until just combined.
Fold in the blueberries carefully to avoid breaking the blueberries and having purple batter, hehe!
In a small bowl, combine the cold butter, brown sugar, and flour with a pastry blender until crumbs form.
To assemble cake, pour half the batter into the greased pan, followed by half the crumb mixture.
Repeat layers making sure to end with the remaining crumb topping.
Place pan in preheated oven for 55 minutes, or until cake is set.
Remove from oven and allow to cool 15 minutes before serving!!