Preheat oven to 350ºF/180ºC/gas 4. Generously spray two 12-cup muffin pans with cooking spray or instead line each cup with paper baking muffin cups.
Mix all dry ingredients first in a mixing bowl.
Add coconut oil and the mashed banana. Stir and while stirring add the milk.
Now add the lemon juice or vinegar. Muffin bater should feel fluffy with a few lumps from the banana.
Last, gently fold in the blueberries. Add the mixture to the 12 cups.
Cooking time is 25 to 30 minutes.
Notes
Frozen or fresh blueberries. When using fresh blueberries muffins can become mushy. Dust the blueberries with a little flour before mixing, to prevent the muffins from turning blue.