Heat the oven to 400°F. Place 1 stick unsalted butter in a 9x9-inch baking dish. Place in the oven while it is heating until the butter melts, 8 to 11 minutes. Remove the baking pan from the oven.

Whisk 2 ½ cups all-purpose flour, ⅓ cup granulated sugar, 4 teaspoons baking powder, and ½ teaspoon kosher salt together in a large bowl. Stir in 2 cups cold buttermilk, 1 teaspoon of the lemon extract, and the finely grated zest of 1 medium lemon with a flexible spatula until a sticky dough forms and there are no pockets of dry flour left. Gently fold in 1 cup of the fresh blueberries.

Transfer the dough into the baking pan. Spread the dough evenly into the pan until it touches all the sides (the dough will be swimming in melted butter). Sprinkle the remaining ⅓ cup blueberries on top. Using the flexible spatula, cut the dough all the way down to the bottom of the pan in a 3x3 grid to make 9 squares (the squares will still be touching each other).

Bake until the biscuits are golden-brown and have puffed up significantly, and a tester inserted into the center biscuit comes out clean, 23 to 25 minutes. Immediately cut along the scores again to separate the biscuits. Let cool for at least 20 minutes in the pan.

Whisk 1 cup powdered sugar, 2 ½ tablespoons lemon juice, and the remaining ¼ teaspoon lemon extract together in a small bowl until smooth. Drizzle over the biscuits and serve.