Pour milk, plant-based or dairy, into a 1 cup measure and add vinegar. Set aside to let milk sour while you proceed with the recipe.
Preheat oven to 400 F. Line a muffin tin with paper baking cups (parchment cups work best) or spray with non-stick baking spray.
In a large bowl whisk together flour(s), baking powder, baking soda, sugar and salt.
In a medium bowl or 2-cup measure, combine canola oil, mashed bananas, sun butter, egg (or flaxseed egg), honey, and vanilla.
Make a well in the center of the dry ingredients and pour in the banana mixture and the soured milk. Stir with a large wooden spoon or spatula only until dry ingredients are moistened.
Add walnuts to batter and stir lightly. (Be sure that you have taken your spoon to the bottom of the bowl in order to mix in any dry ingredients lurking underneath your batter.)
Using an ice cream scoop, place a scoop of batter in each cup.
Bake for about 15 minutes, reversing the pan front to back once to ensure muffins are evenly baked.
Remove from oven and cool for 3 to 5 minutes. Remove the muffins from the tin to prevent soggy, steamed muffins.