Slice the mango lengthwise, avoiding the pit. Hollow out the center of the mango, leaving about 1/2" of flesh around the skin to help the mango maintain structure. Set the halved mango aside in a dish to use as a bowl for the posset.
Using a blender or food proccessor, puree the fresh mango until smooth. Stain through a fine mesh strainer to remove any pulp. You should have about 1/2 cup of creamy mango puree!
Combine the mango puree with the coconut milk, honey (if using) and arrowroot stach. Whisk well.
Add the coconut and mango mixture to a pot over medium-low heat. Simmer on low for 3-4 minutes or until thickened. Add the lime juice and vanilla. Stir well and remove from the heat.
Allow to cool slightly before spooning the filling into the hollowed-out mango. Pour any extra mango filling into a small bowl.
Transfer the posset to the fridge and allow to set for 4-6 hours or until chilled and thick. Serve topped with lime zest!
Notes
What to do with the mango pulp: Save the mango pulp to add to a smoothie or yogurt! You can also include the pulp in this recipe, but I recommend straining it to keep it smoother.