Preheat the oven to 350°F. Cut a round parchment paper base for the bottom of the cake pan(s). Grease an 8- inch cake pan with coconut oil or olive oil spray, place the parchment paper round inside, and set aside.
In a large bowl, whisk together the flour, almond meal, cocoa powder, baking powder, and salt.
In a separate large bowl, combine the fully mashed avocado and granulated sugar. Slowly stir in the water, coconut oil, and vanilla extract.
Slowly incorporate the flour mixture into the wet ingredients, stirring until the avocado is completely smooth. In a small bowl or measuring cup, combine the baking soda and apple cider vinegar and let them fizz. Add to the final cake batter. Batter will be thick.
Pour the cake batter into the prepared cake pan, and bake on the middle rack for 20 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan, then transfer to a plate or cake stand.
While the cake is baking, make the icing. Using a stand mixer or hand mixer, beat together the confectioners’ sugar, chocolate-hazelnut butter, and cocoa powder until smooth. Slowly add the almond milk and continue beating until combined.
When the cake is completely cool, spread the icing evenly across the top and sides using an offset spatula.
When baking with cocoa, remember this smart tip: Baking powder helps to retain flavanols otherwise lost in baking.