Preheat the oven to 450 degrees F (230 degrees C). Place butter in a square baking dish and microwave on High until melted.
Whisk together flour, sugar, baking powder, salt, and lemon zest in a bowl. Add blueberries and toss to coat. Stir in 2 cups buttermilk until just blended and pour mixture into the baking dish with the melted butter.
Use a knife to score the batter into 9 to 12 squares.
Bake in the preheated oven until biscuits are evenly browned and cooked through, 27 to 30 minutes. Let cool for 5 minutes.
Meanwhile, combine confectioner’s sugar, 1 tablespoon buttermilk, and lemon juice in a bowl; whisk together until smooth. Pour glaze on top of the warm biscuits and spread out evenly.
