In a large bowl or stand mixer, cream together butter or margarine, yogurt, and sugar. Add eggs one at a time, beating after each. Mix together flour and cereal separately in a second bowl, and buttermilk and baking soda in a third bowl. Add wet (third bowl) alternating with dry (second bowl) to the first mixture, then stir in nuts, berries, and chips (if using). Cover and refrigerate at least one day and up to two weeks.
Bake as many as desired, as needed, at 400 degrees for 15-20 minutes. Need not be at room temperature to bake.
Yield: 40-50 muffins