In a medium bowl, mix together flour, brown sugar, cinnamon, and salt for the crumble topping
Add the cold diced butter and work it into the mixture with your fingers until crumbly and chunks form
Place the crumble topping in the refrigerator while you prepare the batter
Preheat oven to 350°F and line an 8x8-inch pan with parchment paper
In a medium bowl, whisk together flour, baking powder, and salt for the batter
In a large bowl, beat the butter, granulated sugar, and brown sugar until light and fluffy
Add eggs one at a time, mixing well after each addition
Mix in the vanilla and lemon zest
Mix in half the dry ingredients, then the sour cream, then the remaining dry ingredients until just combined
Toss blueberries with 1 Tbsp flour, then gently fold them into the batter
Spread the batter evenly into the prepared pan and sprinkle the crumble topping evenly over the surface
Bake for 40-45 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs
Cool completely in the pan before glazing or slicing
Whisk together powdered sugar, milk, and vanilla until smooth for the glaze
Drizzle the glaze over the cooled bars before slicing and serving
