Preheat oven to 350 degrees F, prepare a 9"x13" baking pan with non-stick coating.
Combine 1 cup softened butter with 1 cups granulated sugar, add egg, lemon zest and lemon juice, and 1 teaspoon vanilla. Beat until fluffy.
Add 1 cup of chocolate pieces and beat in 2 cups of flour.
Spread evenly over pan. Evenly distribute blueberry preserves or jam over batter, leaving ¼" border.
Evenly distribute fresh blueberries over layer of preserves/jam/
In another bowl, combine 1 cup flour, ¼ cup sugar, 6 tablespoons of chilled butter, cut with two knives or pastry butter until crumbly. Add remaining 1 cup chocolate pieces and mix well.
Evenly distribute crumple topping over blueberries.
Bake in oven for 40-45 minutes until golden brown. Allow to cool completely on metal rack, then cut.
Combine powdered sugar, vanilla, and milk in small bowl, mix well and drizzle over bars, allow to set before serving.
May substitute 2 cups chocolate chips for chocolate bars Batter will be thick, to help spread evenly, lightly rinse palms of hands with water and gently pat down. Can also dip tablespoon in water, shake off, and use bottom of spoon to spread batter.