Preheat the oven to 375°F.
To make the streusel topping:
Mix the sugar, flour, cinnamon, and salt in a small bowl.
Soften butter and mix with the dry ingredients. For a lighter texture, freeze the butter, then grate with a hand held grater over the dry ingredients and blend.
Stir in granola. Set aside.
To make the cake:
Lightly butter an 8" square, 9" square, or 9" round cake pan. Make sure it's at least 2" deep.
Blend the flour, baking powder, and salt together in a medium-sized mixing bowl.
In a separate bowl, beat together the sugar, butter, egg, and vanilla.
Alternately add the buttermilk and the flour mixture to the sugar/butter mixture, ending with flour. Stiffer dough is denser buckle. Add more buttermilk if lighter texture is desired.
Add blueberries and stir only enough to blend.
Spoon the batter into the prepared pan. Sprinkle the topping over the batter.
Bake on the center rack of the oven.
After 30 minutes, check. If top is browned enough, cover loosely with tin foil and bake for another 10 - 15 minutes, or until a toothpick inserted comes out clean.
Cool buckle and enjoy.