Make the streusel topping: Combine brown sugar, flour, and cinnamon, and salt in a small bowl. using your fingers,mix until no lumps remain. Rub in the butter with your fingertips until it becomes crumbly. Refrigerate until ready to use.
Preview the oven to 375 and set oven in the neutral position. Grease a 9 inch square pan with butter or nonstick cooking spray.
In a medium bowl, combine flour, baking powder and salt. Set aside.
In the bowl of an electric mixer, beat the butter and granultaed sugar until creamy, about 2 minutes. Add the eggs one at a time, scraping down the sides of the bowl and beating well after each addition. Beat in the valilla extract and zest.
Gradually add in the flour mixture, alternating it with milk, beating on low speed to combine. Add the berries to the batter and fold gently with a spatula until evenly distributed. Do not over-mix.
Transfer the batter to the preapred pan and spread evenly. Sprinkle the streusel topping over the batter. Bake for 40 to 45 minutes, until golden brown around the edges and a cake tester comes out clean.
Let cool in the pan on a rack for about 20 minutes, then serve right from pan.