Preheat oven to 400º. Butter twelve muffin cups and edges surrounding the cups.
In a large bowl, stir together flour, brown sugar, baking powder, baking soda, and salt. In another bowl, stir together buttermilk, oil, egg, vanilla, and orange peel. Make a well in the center of dry ingredients; add milk mixture and stir just to combine. Stir in cranberries and nuts.
Spoon batter into prepared muffin cups. Bake for 15 to 20 minutes or until a toothpick inserted in center of one muffin comes out clean.
Remove muffin pan to wire rack. Cool for 5 minutes before carefully removing muffins from cups; finish cooling on rack. Serve warm or cool completely and store in airtight container at room temperature.