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  1. Preheat the oven to 350°F. Lightly grease a 12-cup muffin pan.

  2. Beat together the butter and sugar until smooth.

  3. Mix in the mashed banana, then the egg, vanilla, and milk. Scrape the bottom and sides of the bowl and beat briefly to recombine.

  4. Add the baking powder, baking soda, salt, and flours, stirring until smooth. Stir in the caramel chips and walnuts.

  5. Scoop the batter into the prepared muffin pan, mounding it slightly in the wells. (A muffin scoop is a handy tool here.)

  6. Bake the muffins for 20 to 23 minutes, until a toothpick or paring knife inserted into the center of one of the middle muffins in the pan comes out clean.

  7. Remove the muffins from the oven. Carefully tilt each one in its well, to prevent them from steaming as they cool. As soon as you can safely handle them, transfer the muffins to a rack to cool completely.

  8. Store leftover muffins, well wrapped, at room temperature for up to three days; freeze for longer storage.