Place all of the topping ingredients in a small bowl. Use a fork to mix everything together until no dry bits of flour or sugar remain. Set aside.
Grease an 8-inch (20-cm) square baking dish with 1 tablespoon of butter. Mix 1 tablespoon of sugar with ½ teaspoon of cinnamon in a small bowl. Dust the entire inside of the greased baking dish with the sugar-cinnamon mixture. Set aside.
In a large bowl, mix the sugar, butter, egg, milk, and vanilla together until well combined. Sift in the flour then add the baking powder, cinnamon, and salt. Continue to mix until the flour is just combined. The batter will be very thick.
Switch to a wooden spoon or rubber spatula. Add the blueberries to the cake batter and gently fold them in until well distributed. Spoon the cake batter into the prepared pan. Use an offset spatula or butter knife to gently spread the batter evenly into the dish. Scatter the brown sugar topping evenly over the buckle, then cover the baking dish with aluminum foil, and bake for 30 minutes.
Remove the foil and continue to bake the blueberry buckle for 25 to 28 minutes, or until the topping is golden, the blueberries are bubbling, and the sides of the cake are pulling away from the baking dish. Transfer the buckle to a wire rack and cool for 15 minutes before serving warm with ice cream or whipped cream.