Preheat oven to 350° F. Line an 8X8 inch baking pan with aluminum foil. Spray with non-stick spray and set aside.
In a medium bowl, cream together butter and sugar, about 1-2 minutes. Add salt, eggs and vanilla and continue stirring until fully incorporated. Add flour and baking powder and mix until just combined. Reserve ½ cup of dough. Press remaining dough into prepared baking dish.
Make filling: Drain liquid from Blueberries. Set Blueberries aside. Heat juice with sugar over medium heat until sugar is dissolved and small bubbles form at sides of pan. Add a small amount of water to cornstarch and mix well. Add cornstarch mix slowly and stir constantly until liquid is thickened. Add Blueberries and lemon and stir well enrobing blueberries in sauce.
Top dough with Blueberry filling. Drop remaining batter in small pieces evenly over the Blueberry filling.
Place in oven and bake for 50-60 minutes or until golden brown. Remove from the oven and place pan on a wire rack to cool completely. Once cooled, prepare the glaze. Whisk together confectioner’s sugar and milk until smooth and drizzle over the bars. Cut into 12 squares and serve.