Preheat oven to 375˚F. Line an 8x8 pan with foil and grease with butter or cooking spray.
In a medium bowl, toss ingredients for the blueberry layer together until the blueberries are evenly coated. Set aside.
In a large bowl, whisk together flour, 1/2 cup brown sugar, and baking powder for the bottom layer. Using your fingers or two knives, cut in butter and yolk until combined. The texture should be like wet sand.
Press two-thirds of the mixture into the bottom of the prepared pan. Top with the blueberry layer.
Add 2 Tbsp brown sugar to the remaining flour mixture. Crumble this topping over the blueberry layer.
Bake 45 minutes, or until the top is golden brown. Cool completely.
Once the bars are cool, make the cream cheese drizzle. Beat cream cheese until fluffy, approximately 4 minutes. Add sugar and beat until fully incorporated. Add salt, lemon juice and whole milk until combined.
Drizzle over cooled bars and slice. Serve.