3cupsfresh or frozen blueberries (if using frozen, do NOT thaw)
Preheat oven to 375F degrees. Grease a 9-inch square baking pan.
In medium bowl, combine all topping ingredients. Mix together with a fork until it's crumbly. Set aside
In another bowl, whisk together the flour, baking powder, cinnamon and salt.
In large bowl, beat together the remaining ⅓ cup butter and sugar, with an electric mixer until light and creamy. Beat in the egg and vanilla. Add ½ the flour mixture and beat until incorporated. Beat in the buttermilk and then the remaining flour mixture just until combined.
Fold in the blueberries just until incorporated.
Spread the batter evenly into the prepared pan and then sprinkle the crumb topping evenly onto the batter.
Bake until a toothpick inserted in the center comes out clean, about 50 to 60 minutes.
Remove and place pan on a wire rack to cool completely.
Serve as is or with whipped cream or ice cream, if desired.