Preheat the oven to 350°F. Butter 2 x 6" cake pans, dust with flour, and tap out the excess.
In a medium mixing bowl whisk together the sugar, flour, baking powder, cocoa (if using), salt, cream of tartar, and xanthan gum.
Combine the beets, buttermilk, vinegar, and lemon juice and blend until smooth. In a separate bowl, whisk the oil, egg, and vanilla until blended. Pour the beet mixture into the oil mixture and whisk until combined.
Add the wet ingredients to the dry ingredients, mixing as you add, until well-combined and no clumps of dry ingredients are left in the bowl. Mix until smooth.
Pour the batter into the prepared pans.
Bake the cakes for 20 minutes, then rotate in the oven and continue to bake until a toothpick inserted into the center of the cake comes out with just a few crumbs.
Remove the cakes from the oven, and let them cool in the pan for 10 minutes. Run a sharp knife around the edges of the pan and turn them out of the pans to cool completely on a rack.
Recipe by oh honey bakes at https://ohhoneybakes.com/gluten-free-chocolate-beetroot-cake/