Put the brown sugar, flour, cinnamon, and salt in a small bowl to make the streusel topping. Mix with your fingertips until there are no more brown sugar clumps. Use your fingertips to rub in the butter until it becomes crumbly. Keep chilled until you're ready to use.

Set an oven rack in the centre and heat the oven to 375°F. Use butter or nonstick cooking spray to grease a 9-inch square baking pan.

Mix the salt, baking powder, and flour in a medium bowl. Set aside.

The butter and granulated sugar should be creamed for around two minutes in the bowl of an electric mixer. One at a time, add the eggs, scraping the bowl's sides as you go and beating well after each addition. Lemon zest and vanilla extract should be blended in.

Alternately add the milk and flour mixture gradually while mixing on low speed to blend. Using a spatula, carefully fold the batter while adding the berries to ensure fair distribution. Avoid overmixing.

Spread the batter evenly after transferring it to the prepared pan. Overtop the batter with the streusel topping in a uniform layer. Bake for 40 to 45 minutes, or until a cake tester comes out clean and the edges are golden brown. After roughly 20 minutes of cooling on a rack in the pan, serve straight from the pan.

The day the cake is created is the finest day to serve it. When refrigerated at room temperature and wrapped in foil, leftovers will last for a few days.