MAKE THE TOPPING: In a small bowl, combine all of the topping ingredients: brown sugar, granulated sugar, cinnamon, and butter.
PREPARE: Preheat the oven to 350˚F and adjust the oven rack to the middle position. Grease an 8x8 pan with butter or oil and set aside.
COMBINE THE DRY INGREDIENTS: In a large bowl, whisk together the flour, baking powder baking soda, cinnamon, and salt.
COMBINE THE WET INGREDIENTS: In a medium bowl, whisk together the sour cream, brown sugar, granulated sugar, vanilla, eggs, and butter.
COMPLETE THE CAKE BATTER: Pour the wet ingredients into the dry ingredients and mix until just incorporated. Add the blueberries and mix just enough to evenly distribute them.
ASSEMBLE TO CAKE: Spread the batter in the prepared dish in an even layer. Sprinkle the topping mixture on top of the cake batter.
BAKE: Bake the cake for 50-60 minutes or until the top is golden brown and the center is almost set. Allow the cake to cool for 30 minutes before icing.
ICING: In a small bowl combine the powdered sugar and milk. Drizzle over the top of the cooled cake and allow to dry for 5 minutes before serving.