Preheat oven to 400°F; line 11 muffin cups with paper liners.
In a medium bowl, whisk together flour, baking powder, baking soda, salt and cinnamon.
In a large bowl, whisk together sugars and the egg until smooth. Beat in vegetable oil and vanilla. Working in 2 or 3 additions, alternate adding the flour and the buttermilk to the sugar mixture, stirring until just combined.
In a small bowl, stir together remaining brown sugar and cinnamon.
Fill each muffin cup about 1/2 full. Sprinkle with a scant 1/2 tsp of the brown sugar mixture into each cup. Evenly divide remaining batter on top and sprinkle with remaining sugar mixture.
Bake at 400°F for 15-18 minutes, until a tester comes out clean.
Transfer to a wire rack to cool completely before eating.