Preheat oven to 350 degrees. Grease muffins pans. In a large mixing bowl, combine the first 10 ingredients. In a separate bowl, cream butter and brown sugar. Add vanilla and buttermilk; mix well. Then add eggs and mix well. Fill prepared muffin pans (only 3/4 full). Bake 20 minutes or until done. Cool.
To make frosting:
Combine ingredients and mix well. Frost cooled muffins.