Best: Basic Muffin Mix: In large bowl, stir together all-purpose and whole wheat flours, bran, milk powder, sugar, baking powder and salt until combined. [for storage: transfer to airtight container; store in cool, dry place. Stir well before using.] Chocolate Banana Muffin: In bowl, stir "Best: Basic Muffin Mix" with chocolate chips. Whisk together banana, water, oil, egg and vanilla; stir into dry ingredients just until moistened. Spoon into greased muffin cups, filling to top. Top each with banana slice. Bake in 375F 190C oven for about 30 minutes or until tops are firm to the touch. Let cool in pan on rack for 5 minutes; remove muffins to rack and let cool completely.