01 - Preheat oven to 350°F. Grease a 9-inch square baking pan and line with parchment paper, allowing an overhang for easy removal.
02 - Whisk together flour, baking powder, and salt in a medium bowl. Set aside.
03 - In a large bowl, beat softened butter and sugar until light and fluffy, approximately 2 to 3 minutes.
04 - Add eggs one at a time, mixing after each addition. Stir in vanilla extract, lemon zest, and lemon juice.
05 - Add the dry ingredient mixture in three parts, alternating with milk, beginning and ending with flour. Stir until just combined without overmixing.
06 - Lightly coat blueberries with 1 tablespoon flour, then gently fold into the batter.
07 - Spread batter evenly in prepared pan. Sprinkle coarse sugar over top if desired.
08 - Bake for 38 to 42 minutes until a toothpick inserted into the center comes out clean and the top is golden.
09 - Let cool in the pan for 15 minutes, then remove using parchment overhang. Cool completely on a wire rack before slicing.