01 - Preheat your oven to 350°F. Thoroughly grease and lightly flour a 9-inch round or square baking pan.
02 - In a medium bowl, combine 1/2 cup all-purpose flour, 1/3 cup packed light brown sugar, 1/2 teaspoon ground cinnamon, and a pinch of salt. Pour in 1/4 cup melted unsalted butter and stir until moist, coarse crumbs form. Set this mixture aside.
03 - In a large mixing bowl, using an electric mixer, cream together 1/2 cup softened unsalted butter and 1 cup granulated sugar on medium speed until the mixture is light and fluffy, approximately 2 to 3 minutes.
04 - Gradually beat in the 2 large eggs, one at a time, ensuring each is fully incorporated before adding the next. Stir in 1 teaspoon vanilla extract, 2 teaspoons lemon zest, and 2 tablespoons lemon juice until well combined.
05 - In a separate bowl, whisk together 2 cups all-purpose flour, 2 teaspoons baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt.
06 - Add half of the dry flour mixture to the creamed butter mixture, mixing gently on low speed until just combined. Next, pour in 1/2 cup whole milk and 1/4 cup sour cream, blending until incorporated. Finally, add the remaining dry flour mixture, mixing only until no streaks of flour remain, being careful not to overmix.
07 - Using a spatula, gently fold 1 1/2 cups fresh blueberries into the batter. If utilizing frozen blueberries, ensure they are unthawed and lightly tossed with 1 tablespoon of flour before adding.
08 - Pour the prepared batter evenly into the greased and floured 9-inch baking pan. Distribute the reserved crumb topping uniformly over the batter.
09 - Bake for 40 to 45 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean, indicating it is fully cooked.
10 - Allow the coffee cake to cool in the pan for 15 minutes. After this initial cooling, carefully transfer it to a wire rack to cool completely.
11 - For an optional glaze, in a small bowl, whisk together 1/2 cup powdered sugar with 1 to 2 tablespoons of lemon juice until smooth and pourable. Drizzle the glaze over the cooled coffee cake just before serving.