Step 1

Make the streusel topping: Combine the brown sugar, flour, cinnamon, and salt in a small bowl. Using your fingers, mix until no lumps of brown sugar remain. Rub in the butter with your fingertips until it reaches a crumbly state. Refrigerate until ready to use.

Step 2

Preheat the oven to 375°F and set an oven rack in the middle position. Grease a 9-inch square pan with butter or nonstick cooking spray.

Step 3

In a medium bowl, whisk together the flour, baking powder and salt. Set aside.

Step 4

In the bowl of an electric mixer, beat the butter and granulated 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 vanilla extract and lemon zest.

Step 5

Gradually add the flour mixture, alternating with the 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.

Step 6

Transfer the batter to the prepared pan and spread evenly. Sprinkle the streusel topping evenly 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 the pan.