Preheat the oven to 375 F and line a muffin tin with 9 unbleached paper muffin cups.
Prepare the flax egg by whisking together the ground flax and water in a small bowl. Set aside for 5 minutes to thicken.
Mash the banana and measure out 1 3/4 cups. Place in a large bowl. Add the coconut sugar and whisk. Add the flax egg, melted coconut oil, and vanilla and whisk to incorporate.
Add the dry ingredients, one at a time, whisking after each addition.
Measure 2 tbsp of batter into each muffin cup. Top each with 1 tsp of vegan chocolate hazelnut spread. Top each with approximately 2 tbsp more batter.
Bake for 20-25 minutes, or until lightly brown and fragrant.