In a small mixing bowl, blend together the brown sugar, flour, cinnamon, and salt until fully combined, ensuring no clumps of brown sugar remain. Using your fingertips, work the chilled butter into the mixture until it achieves a crumbly texture. Place the topping in the refrigerator to keep it firm until needed.

Set your oven to 375°F and position the oven rack in the center. Grease a 9-inch square baking dish thoroughly with butter or a nonstick spray for easy removal later.

In a medium-sized bowl, whisk together the flour, baking powder, and salt until evenly distributed. Set aside.

In the bowl of a stand mixer fitted with a paddle attachment, beat the softened butter with the granulated sugar on medium speed for about 2 minutes, or until the mixture is light and fluffy. Add the eggs one at a time, beating well and scraping down the sides of the bowl between each addition. Incorporate the vanilla extract and lemon zest, mixing until evenly combined.

Reduce the mixer speed to low. Gradually add the flour mixture, alternating with the milk, beginning and ending with the dry ingredients. Beat each addition just until combined, taking care not to overmix the batter.

Gently fold the blueberries into the batter with a spatula, ensuring they are evenly distributed. Be careful not to crush the berries while mixing.

Transfer the batter into the prepared baking pan, spreading it out evenly with a spatula. Evenly sprinkle the chilled streusel topping over the batter. Bake for 40 to 45 minutes, or until the edges are golden and a tester inserted into the center of the cake comes out clean.

Allow the cake to cool in the pan on a wire rack for about 20 minutes before serving. Slice directly from the pan and enjoy warm or at room temperature.