Preheat the oven to 350 degrees F and line a muffin tray with 9 papers.
Add all ingredients (except the optional chopped walnuts) to a blender and blend until combined.
Fill the muffin papers ¾ of the way full with muffin batter. Sprinkle with chopped walnuts. Bake in the preheated oven 25 to 30 minutes, until the tops have set up and turn golden-brown.
Allow muffins to cool at least 30 minutes before peeling off the muffin papers.