Preheat oven to 350 degrees Fahrenheit. Prepare an 8 x 8-inch baking dish with cooking spray; set aside.
In a large mixing bowl, whisk together flour, 1/2 cup sugar, salt, and baking powder until well blended. Stir in milk, butter, and vanilla until well combined. Spread in prepared baking dish.
Sprinkle blueberries and sugar evenly over batter.
Bake for 45-50 minutes or until a toothpick entered in the center of the cake comes out clean.