Preheat oven to 350°. Grease a muffin tin, including the top and insert paper liners if desired.
Mix together the flour, salt, baking powder and baking soda in a medium bowl.
Whisk the egg with the buttermilk and vanilla and set aside.
Cut the butter into 8-10 pats and pulse with the sugar in a food processor until mixed but still lumpy, about 12-15 times. (The butter particles should be slightly smaller than pea size)
Add the milk and egg mixture and pulse a 5-6 times until mixed well but still lumpy.
Add the dry ingredients to the food processor bowl. Pulse 3-4 times, just until the dry ingredients are moistened.
Scrape the batter into a large bowl and gently fold in the blueberries.
Evenly divide the batter among the muffin cups and sprinkle with the cinnamon-sugar.
Bake about 25 minutes or until deep golden brown on top.
Cool slightly before removing from the muffin tin.