Preheat oven to 350 degrees Fahrenheit. Line a 12-cup muffin tin with liners.
Mix all ingredients together in a medium bowl until very smooth. Fold in optional add-in.
Using an ice cream scoop, divide batter evenly into muffin cups.
Bake for 15 minutes. Muffins should be firm.
Cool for 15 minutes before removing from pan.