Stir brown sugar and cinnamon together in small bowl. Add butter; stir until blended. Set aside.
Cake
Lightly grease and flour 9 x 13-inch (23 x 33 cm) baking pan. Set aside.
Beat butter and sugar in large bowl with electric mixer until blended. Add eggs and vanilla; beat until blended.
Stir flour, baking powder and salt together in medium bowl. Add a third of flour mixture to egg mixture; beat just until blended. Add half of milk mixture, beat just until blended. Continue alternating flour mixture and milk until both are incorporated into batter. Add blueberries; mix batter gently by hand just until combined. Spread batter into prepared baking pan. Sprinkle topping evenly over batter.
Bake in preheated 350ËšF (180ËšC) oven until toothpick inserted near centre of cake comes out clean, 40 to 45 minutes.
 Notes
This cake is especially delicious made with wild blueberries.
For a nut topping, replace 1/3 cup (75 mL) sugar with 1/4 cup (50 mL) all-purpose flour, and add 1/2 cup (125 mL) sliced almonds or finely chopped walnuts.