In a measuring cup, stir together the non-dairy milk and apple cider vinegar to create the buttermilk. Set aside.
Using a fork, mash the banana in a large mixing bowl.
Add the runny peanut butter and brown sugar to the bowl - stir to incorporate.
Stir the vanilla and buttermilk into the bowl ingredients.
Measure the flour, baking powder, cinnamon, nutmeg, and salt into the bowl.
Fold the dry ingredients into the wet ingredients just until incorporated.
Chop the walnuts to desired size and stir into the batter.
Divide the batter evenly among the prepared 6 muffin cups.
Cut 6 slices off a banana (if you have an extra) sprinkle with brown sugar. Poke one slice into each muffin top then add a sprinkle of chopped walnuts.
Additional topping ideas in recipe notes below.