Preheat your oven to 375°F (190°C) and grease a 9×13 inch baking pan.
In a large mixing bowl or stand mixer, cream together the granulated sugar and shortening until light and fluffy.
Add the eggs one at a time, beating well after each addition, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the creamed mixture, alternating with the milk, until just combined.
Gently fold in the 3 cups of blueberries by hand.
In a small bowl, combine the brown sugar and cold cubed butter using a fork or pastry cutter until the mixture resembles coarse crumbs.
Spread the batter into the prepared baking pan.
Sprinkle the streusel mixture evenly over the top of the batter.
Bake for 40 minutes or until a toothpick inserted into the center comes out clean.
Allow to cool before slicing and serving.