Preheat your oven to 375 degrees
Grease a 9-inch square baking pan using non-stick cooking spray or butter
Begin making the streusel topping by mixing together the brown sugar, flour, cinnamon, and salt in a small bowl.
Using your fingers, mix in the cold butter until it resembles a crumble texture. Place in the fridge until ready to use
In a medium bowl, combine the flour, baking powder, and salt and set aside.
Using an electric mixer with a beater attachment, beat the butter and granulated sugar together until creamy (about 2-3 minutes)
Add in the eggs one at a time, making sure to beat well after each addition
Beat in the vanilla extract and lemon zest until combined
Alternate between adding the flour mixture and milk into the butter mixture, and beat on low speed until combined (making sure to not overmix the batter)
Using a rubber spatula, gently fold in the blueberries until combined throughout the batter
Pour the batter into the prepared pan, and use the spatula to create an even layer (the batter will be thick)
Sprinkle all the streusel topping over the batter and bake in a preheated oven for 40-45 minutes.