In a small mixing bowl, combine the flour, baking powder, salt, and cinnamon. Whisk together to break up any clumps. Set aside.
In a large mixing bowl, beat together the oil, eggs, sugar, buttermilk, and vanilla until smooth.
Pour the dry ingredients into the wet ingredients and mix just until combined. Set aside.
Add the flour, sugar, and cinnamon for the streusel to a medium mixing bowl and stir together. Use a fork to cut the butter into the dry ingredients until it resembles sand. (see note)
Spray the cups of two muffin tins with cooking spray or line them with cupcake liners.
Fill each muffin tin cup halfway with batter. Sprinkle half of the cinnamon streusel over the muffin batter in each cup. (This will give a cinnamon swirl in the middle of each muffin.)
Use the remaining batter to fill the muffin cups the rest of the way. Then sprinkle the remaining streusel on top of the muffins.
Bake at 425 degrees F for 5 minutes. Then reduce the oven temperature to 350 degrees F (without opening the oven) and bake for another 11-12 minutes, or until a toothpick inserted in the center comes out clean.
Remove from the oven and allow the muffins to cool for 5 minutes in the pans. Then transfer them to a wire rack to cool completely. (They're great warm too!) Store any leftovers in an airtight container.
Notes
If the streusel seems too dry, add another tablespoon of butter. The crumbles should stick together when you pinch them with your fingers.