Heat the oven to 350°F. Line the bottom and two opposite sides of a 8x8-inch baking pan with a sheet of parchment paper long enough to hang over the sides by 1 inch to form a sling.

Rub 1 cup granulated sugar and the finely grated zest of 1 medium lemon together in a large bowl with your fingers until fragrant. Whisk in 2 room-temperature large eggs, 1 stick melted and cooled unsalted butter, 1 teaspoon lemon extract, and ½ teaspoon kosher salt until combined.

Add 1 cup all-purpose flour and sprinkle with ½ teaspoon baking powder. Fold together with a flexible spatula until just combined. Fold in ¾ cup of the fresh blueberries.

Transfer the batter to the baking pan and smooth into an even layer. Top evenly with the remaining ¼ cup fresh blueberries.

Bake until browned, the edges pull away from the sides of the pan, and a tester inserted into the center comes out clean or with just a few moist crumbs, about 30 minutes. Do not overbake. Let cool completely, about 1 ½ hours.

Stir 1 ounce room-temperature cream cheese and ¼ cup powdered sugar together in a medium bowl until combined. Whisk in 1 teaspoon lemon juice until you have a smooth, pourable glaze. Spread the glaze evenly over the bars. Let sit until the glaze sets, about 15 minutes.

Grasping the excess parchment, lift the slab out of the baking dish and place on a cutting board. Cut into 12 pieces.