1. Preheat oven to 375°F. Grease a 9-inch cake pan and set aside.
2. To make the topping: In a small bowl, mix together brown sugar, flour, and cinnamon. Rub in the butter using your finger tips until the mixture is crumbly. Set the topping aside.
3. To make the batter: In a medium bowl, whisk together flour, baking powder, and salt. In the bowl of a stand mixer, cream together the sugar and butter. Add the egg and vanilla, mix until smooth. Alternately add the buttermilk and the flour mixture to the creamed mixture, ending with flour. Stir only enough to blend. Gently fold in the blueberries.
4. Pour the batter evenly into prepared cake pan. Sprinkle the topping over the batter and bake for 40 to 45 minutes or until a toothpick comes out clean. Let the buckle cool for 10 minutes. Loosen the sides with a knife. Holding the cake pan in your left hand, gently tip the cake out onto your right hand, remove the pan, and gently place the cake onto a serving dish.