Preheat oven to 325 degrees and lightly grease a muffin tin.
Empty avocado flesh into a small bowl. Mash with a fork until completely smooth.
In a large bowl, cream together softened butter and granulated sugar. Add egg, vanilla, mashed avocado, and lime juice, and stir well to combine. Set aside.
In a medium bowl, gently combine flour, baking powder, baking soda, cinnamon, and salt using a fork.
Fold the dry ingredients into the wet ingredients until just combined. Avoid over-mixing as this will toughen the batter.
Spoon batter into the muffin tin, filling each well about ¾ of the way.
Lightly sprinkle muffin tops with sugar before baking.
Bake for 18-20 minutes, or until an inserted toothpick comes out clean. Let muffins cool for at least 5 minutes before removing from the pan.
Serve warm, or store in an air-tight container for up to 5 days.*