Preheat oven to 375°F. Grease an 8-inch baking pan and set aside.
Beat sugar and butter together with an electric mixer on medium until blended. Beat in eggs and vanilla.
In another bowl, combine flour, baking powder, salt and cinnamon.
Stir flour mixture and buttermilk into the sugar mixture until well blended.
Stir in blueberries and spoon the batter into the baking pan.
For the Streusel topping, combine all the ingredients except the butter. Add butter in and mix until it becomes crumb like. Sprinkle the streusel evenly over the batter.
Bake 30 to 35 minutes or until a toothpick comes clean from the centre of the cake. Let cool before cutting into slices.
Notes
Adding a teaspoon of flour to your fresh blueberries will help them from sinking to the bottom of the cake. Just mix four around and dust off any excess before dumping into the batter.