Roast or cook beets, peel and puree.
Preheat oven to 180 degrees. Coat two 8-inch cake pans with cooking spray or line with parchment paper. Set aside.
Whisk together almond milk and cream of tartar in a large bowl. Let sit about 5 minutes to curdle.
Add 1 cup of beet puree, sugar, coconut oil, and vanilla extract to the almond milk mixture. With a hand mixer, beat until foamy, about 30 seconds. (This is important so that the coconut oil does not separate and solidify.)
Sift in remaining dry ingredients. Beat again to incorporate.
Divide between the cake pans. Bake at 180 until a toothpick inserted in the centre comes out with fudgy crumbs, approx. 30 mins.
Allow the cakes to cool completely before removing, or they will not hold together!
Scoop out the flesh of the avocados and place in the bowl of a food processor. Puree until smooth. Add remaining ingredients and puree again, scraping down the sides of the bowl.
Once cooled spread with half the icing, and top with the second cake layer. Spread on the rest of the icing and add any decorations you like!
Refrigerate before serving. The cake can be served immediately, but we like it best after being in the fridge overnight.