Preheat oven to 180 ºC and line a 12-cavitiy cupcake pan with paper liners.
In a large mixing bowl sift together spelt flour, potato starch, psyllium husk powder, vanilla powder, baking powder, baking soda and a small pinch of salt. Mix with a wire whisk until combined and set aside.
In a separate bowl vigorously whisk the milk, apple cider vinegar, olive oil and cane sugar. Pour this wet mixture into the bowl with the dry ingredients and mix until no large lumps remain.
Fill cupcake liners 2/3 full (so they rise evenly) and bake for 20 to 22 minutes.
Carefully unmould and transfer to a cooling rack so they cool completely.
Soak the cashews in 2 cups of water with a small pinch of salt overnight.
Rinse well and place the cashews into your high-speed blender or food processor with the almond milk. Blend really well until you get a smooth cream with no lumps, you might need to stop and scrape the sides of the blender jug or bowl a few times. Add in the agave syrup, vanilla powder, lemon juice and salt. Blend again until completely combined and gradually pour in the melted coconut oil and cacao butter. The buttercream should look smooth and creamy.
Finally add in a small amount of blue spirulina powder until you get a nice pastel blue tone.
Transfer the cashew buttercream to an airtight container, cover the surface with plastic wrap, put the lid on and refrigerate until the cream sets and thickens. If it’s too hard right before frosting the cupcakes let it thaw at room temperature until it has a pipeable consistency.
Frost your cupcakes with the blue buttercream, top with popcorn. Store cupcakes covered in the refrigerator.