Mix-ins such as chopped walnuts, pecans or raisins can be added to the batter just prior to pouring batter into pans. This recipe can accommodate about one and one half cups of mix-ins.
As a rule I mix wet ingredients and dry ingredients separately. However I have mixed all ingredients in the same bowl without noticing a difference in the final product. To use a single bowl, mix wet ingredients first followed by the dry ingredients.