Preheat oven to 350 degrees, spray an 8 in square baking pan with cooking spray with our without flour in the spray, set aside .In a large mixing bowl, whisk together flour, sugar, and salt .In a glass measuring cup or small mixing bowl, combine the egg, lemon juice, zest, and vanilla extract. Pour that into the large mixing bowl along with the softened butter. Mix well. Make sure you get all the dry mix incorporated. Try not to over mix. Toss the 1 tablespoon of flour with the blueberries and toss gently into batter. Fold in very carefully. Pour batter into prepared baking pan. Bake for about 22-25 minutes. Should slightly bounce back when lightly touched on top and toothpick should come out mostly clean. Let cool at least 20 to 30 minutes. Sprinkle with powdered sugar. Cut into pieces and enjoy!