Preheat oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper or lightly spray with non-stick cooking spray.: As the oven warms, you will notice a dry, warm scent filling the kitchen, signaling an even bake environment; preheating ensures the cookies start cooking immediately for proper rise and texture. Using parchment prevents sticking and promotes even browning on the bottom, while a light spray can be a fallback if you do not have parchment. A common mistake is putting cold dough into an oven that is not fully preheated, which can cause uneven spread and inconsistent texture, so wait until the oven reaches the full temperature.
Using a hand or stand mixer, whisk together the 1 cup sugar, 1/2 cup softened unsalted butter, and 4 ounces cream cheese until smooth and creamy. Add in 1 egg and 1 teaspoon vanilla and beat until just incorporated.: You will hear the mixer change tone as the ingredients come together, and the bowl contents should look glossy and homogeneous, with no streaks of butter or lumps of cream cheese . Creaming traps air tiny pockets, which contributes to a tender structure; achieving a smooth emulsion here is important for consistent crumb. Overbeating at this stage can make the batter too soft and cause excessive spreading, so mix until combined and smooth, pausing to scrape the bowl so everything is evenly incorporated.
In another bowl combine 2 1/2 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon baking powder, and 1/4 teaspoon salt. Beat the flour mixture into the butter mixture until incorporated. Gently fold in the 2 cups blueberries.: When you whisk the dry ingredients together, you will see a uniform, pale mixture that ensures even distribution of leavening and salt, which prevents pockets of bitter or overly salty spots in baked cookies. Sifting or thorough whisking aerates the flour slightly, which helps keep the cookies light. A common error is adding wet ingredients before properly mixing the dry ones, which can lead to uneven rising and spots of unmixed leavening in the final bake.
Drop heaping tablespoons of dough onto the cookie sheet. Bake for 10-12 minutes. Let cool on a wire rack.: After adding the dry mixture you will notice the dough thicken and become more structured, moving from glossy to more matte as the flour hydrates. Folding in the blueberries should be gentle and brief, because their skins are delicate and you want whole bursts of fruit rather than a uniformly purple dough. If you overmix, the dough will become tough and the berries will bleed, so stop once the berries are evenly distributed.
Whisk together the 1 1/2 cup powdered sugar, 1 tablespoon lemon juice, 1 tablespoon milk, 1 teaspoon vanilla, and the zest of one lemon. Drizzle on top of cookies.: As the cookies bake, watch for edges to set and become lightly golden while the centers remain soft; you may hear a faint crackle as moisture escapes. The tops should look mattified rather than wet. Cooling on a wire rack allows air to circulate underneath, preventing the bottoms from becoming soggy. A frequent misstep is leaving cookies on the hot sheet too long after baking, which continues cooking them and can rob them of a soft center, so transfer to the rack within a couple of minutes.
Whisk together the 1 1/2 cup powdered sugar, 1 tablespoon lemon juice, 1 tablespoon milk, 1 teaspoon vanilla, and the zest of one lemon. Drizzle on top of cookies: The glaze should be smooth and glossy, with a bright citrus aroma from the lemon zest . Drizzling while the cookies are slightly warm allows the glaze to set thinly on the surface, creating a pleasant contrast between soft cookie and slightly firmer topping. If the glaze is too thin, it will run off the cookies and pool on the rack, so adjust by adding more powdered sugar a little at a time until it reaches a syrupy consistency. A common oversight is using too much liquid, which makes a runny glaze and reduces the visual impact and texture contrast.