FOR MUFFINS: Preheat oven to 375°.
Spray a muffin tin with cooking spray or paper liners.
In a large bowl, whisk together the flour, sugars, baking powder, salt and cinnamon.
In a separate bowl combine the buttermilk, eggs, vanilla, and melted butter and whisk together.
Pour the wet ingredients into the dry ingredients and mix gently just until combined, being careful not to overmix.
Gently fold blueberries into the batter.
Fill muffin cups to top with batter; if using streusel topping, fill the cup ¾ full and fill to top with streusel.
Bake in the preheated oven for 15-20 minutes, until slightly brown and a toothpick inserted in the center of the muffins comes out clean.
FOR LOAF: Preheat oven to 350°.
Spray a 9X5 inch loaf pan with cooking spray.
Follow the same directions as the muffins.
Pour into the loaf pan, top with streusel and bake for 45-60 minutes, until slightly brown and a toothpick in center comes out clean.
Chef's notes: You can substitute buttermilk by adding 1 Tablespoon of lemon juice in a liquid measuring cup and fill to the 1 cup mark with milk. Streusel Topping (makes about 3 cups): Combine 1/2 cup brown sugar (lightly packed; can use light or dark), 1/2 cup granulated sugar, 2 cups flour, 3 teaspoons cinnamon, 1/4 teaspoon salt, 1 cup unsalted butter (cut into small pieces, cold), and 1 cup pecans (chopped, optional). Combine sugars, flour and spices. Add cold butter pieces and mix in with pastry cutter or fingers until mixture resembles coarse meal. Sprinkle on top of whatever filling you are making. Can be made in advance. Store in an air tight container for up to 3 days prior to use.