Preheat the oven to 350℉. Grease and line two 9-inch round cake pans with parchment.
Rinse and scrub the beets well to remove any dirt or debris. If you clean the skin well, there is no need to peel them. Use the finest side of a box grater to grate the raw beets into a bowl. The number of beets you will need will depend on their size, but 2 medium beets should be enough.
In a large mixing bowl, whisk together the spelt flour, cocoa powder, baking soda, and salt.
Transfer the grated beets to a large mixing bowl along with their juices. Crack the eggs into the bowl and add the oil, sugar, and vanilla extract. Whisk everything together until completely combined. Pour in the buttermilk and whisk again to incorporate.
Pour the dry ingredients into the mixing bowl with the wet ingredients. Using a baking spatula, fold the ingredients together until just combined and no spots of dry flour remain. Fold in the white vinegar.
Divide the cake batter evenly between the two lined cake pans. Bake the cakes for 30 minutes or until a toothpick inserted in the centre of the cakes comes out clean. Remove the cakes from the oven and allow them to cool in their pans for 10 minutes before flipping them out and transferring them to a cooling rack to cool completely.
Prepare the cream cheese icing. Add the softened cream cheese and salted butter to the bowl of a stand mixer affixed with the paddle attachment. Alternatively, you can use a handheld mixer and a large mixing bowl. Beat the cream cheese and butter on medium until light and fluffy.
Add the icing sugar to the butter and cream cheese mixture in increments so it doesn't go all over your kitchen. Add a bit of the icing sugar, beat it in, then add more. Three additions seem to be a sweet spot.
Once all the icing sugar has been added, beat in the vanilla. Beat the icing on medium speed for about a minute, or until it is smooth, light, and fluffy.
Make sure the cakes have completely cooled before you ice them. I usually will bake my cakes a day in advance, wrap them, and store them in the fridge before icing them the next day. Stack the two cake layers with a layer of cream cheese icing in between, then ice the cake, decorating as you please.
Store the cake in the fridge until you are ready to serve it, then slice and serve! Leftover cake will keep in the fridge for five days.