Preheat oven to 350-degrees. Line 18 muffin tins with (unbleached) muffin liners.
In a high-speed blender, combine all ingredients except chocolate chips. Blend until smooth.
Stir in chocolate chips.
Spoon batter into muffin cups using a 2" cookie dough scoop or 1/4-cup measure.
Bake in the preheated oven for 18 minutes (19 minutes if using spinach) or until a toothpick inserted into the center comes out clean.
Cool completely before serving. Store in an airtight container for up to 3 days, or freeze for up to 1 month.