Preheat oven to 400 degrees.
Place all ingredients (except add-ins) in a blender.
Blend batter until smooth; will be sticky because of the nut butter.
Grease muffin tins with cooking spray. Add add-ins (my Vitamix heated the batter so cool a bit if you're using chocolate chips as they will melt) and pour/scrape batter into small or mini muffin tins (makes 12 minis and 8 small [my size] muffin molds).
Bake in the 400-degree oven for about 10 minutes (my tin size/oven took about 15 minutes) until muffins are well-browned and a toothpick comes out clean from the center.
Cool at least 10 minutes in pan and then turn out on plate or wire pack to cool completely. You can store these in a closed container in the fridge for at least a week (but they'll most likely be gone by then). ENJOY!