Streusel topping: combine the brown sugar, flour, cinnamon and salt in a small bowl. Use a fork to cut the butter into the other ingredients. Then use your fingers to combine completely. Set aside.
Preheat the oven to 375 degrees. Grease a 9 inch square pan with butter or nonstick cooking spray.
In a medium bowl, mix the flour, baking powder and salt. Set aside.
Then in the bowl of an electric mixer (or a large mixing bowl use a hand mixer), beat the butter and granulated sugar until creamy.
Add the eggs one at a time, beating well after each addition.
Next add in the vanilla and lemon zest and mix well.
Gradually add the flour mixture, alternating with the milk, beating on low speed to combine.
Once those ingredients are combined well, fold in the berries to the batter.
Pour the batter into the prepared pan and sprinkle the streusel topping evenly over the top.
Bake for 40-45 minutes or until golden brown around the edges and the middle has set.
Cool for 20 minutes then serve.