Begin by preheating your oven to 350 degrees.
Place paper liners or coat a cupcake pan with non-stick spray.
In a large mixing bowl, combine the flour, sugar, flaxseed (optional), baking soda, baking powder, and salt. Mix well with a large spoon.
Next, add the mashed avocado, egg, vanilla and greek yogurt. Mix with a hand mixer until well blended.
Use a large spoon and stir in the chopped prunes.
Fill your muffin cups about three-fourths full. Bake at 350 degrees for 25 to 30 minutes or until a toothpick comes out clean from the middle.
Serve warm with butter on top (if desired).