In a small bowl, combine the flour, baking powder, salt, and cinnamon. Whisk to break any clumps.
In another bowl, combine the oil, eggs, sugar, buttermilk and vanilla. Mix well.
Pour the wet ingredients into the dry ingredients and mix just until combined.
For the Streusel:
In a small bowl, combine the sugar, cinnamon, and butter, cutting it with a fork until it resembles coarse crumbs.
Spray 2 muffin pans with cooking spray.
Fill 14-16 muffin cups half full.
Sprinkle half of the cinnamon streusel on over the muffin batter.
Use the remaining batter to fill the muffin cups the rest of the way.
Sprinkle the remaining streusel on top of the muffins.
Bake at 425 for 5 minutes, then reduce the oven temperature to 350 degrees (without opening the oven) and bake for an additional 11-12 minutes, or until the center of the muffin bounces back when you touch it.
Allow the muffins to cool for 5 minutes, then remove them from the pan and allow them to cool on a wire rack.