Position a rack in the center of the oven and preheat to 350ºF. Grease a 6-inch round cake pan with softened butter and line the bottom with a round of parchment paper cut to fit.
Make the Cake Batter
In a large bowl, whisk together the sugar, oil, egg, beet puree, buttermilk, lemon juice, vinegar, and vanilla.
Place a strainer over the bowl and add the teff, sweet rice, and oat flours with the cocoa powder, baking powder, baking soda, and salt. Sift the flours into the beet mixture and whisk or stir to combine.
Bake the Cake
Scrape the batter into the prepared pan and bake until the cake springs back to the touch and a toothpick inserted near the center comes out with moist crumbs, 35-45 minutes. Remove from the oven and let cool slightly, then invert onto a wire rack, peel away the parchment, and let cool completely. The cake can be stored covered at room temperature for 1 or 2 days.
Make the Frosting
Make the cream cheese frosting as directed. Divide into 4 bowls. Leave one bowl white. To the second bowl, add a big pinch of beet powder. To the third bowl add ¼ teaspoon beet powder. And to the fourth bowl add 1 teaspoon beet powder. Stir each bowl well, adding more color if you like (the color continues to intensify as the frosting sits).
Assemble
Cut the cake into two even layers using a serrated knife. Place the bottom half on a serving plate, cut-side up. Place the top half on a rimless plate or cutting board cut-side down. Layer the frosting into a large piping bag fitted with a large star tip, starting with the white frosting and ending with the most vibrant frosting. Pipe a line of frosting on the bottom cake layer, following the curve of the cake. Repeat with the top cake layer. Return to the bottom cake, and pipe another row of frosting next to the first. Repeat with the upper layer. Continue until both layers are covered in frosting; you should have just enough frosting to cover both layers, though if your frosting is extra fluffy, you’ll have a bit left over.
Carefully place the top cake layer over the bottom cake layer. The cake is best served at room temperature, though it’s easier to cut when cold. Optionally chill the cake until firm, then cut it into wedges using a large chef’s knife dipped in hot water and wiped clean between cuts, then let the slices come to room temperature before serving. Store leftover cake refrigerated airtight for up to 3 days; serve at room temperature.
Notes
To make the beet puree: Scrub 2 raw medium-sized red beets, place in a small saucepan covered by an inch of water, and boil until tender when pierced with a knife, 20-30 minutes or so. Drain, let cool to room temperature, peel, then cut into large pieces and puree in a food processor until smooth. You’ll probably have enough puree for a few cakes. Store in the refrigerator for up to 1 week, or freeze for up to several months.*Substitutions:
For teff flour, sub by weight almond flour or millet flour.
For sweet rice flour, sub by weight cassava flour or GF AP flour such as Bob's Red Mill.
For oat flour, sub by weight almond flour or sorghum flour.
If you only have natural cocoa powder, it will work here, but the cake will have a more acidic, beet-forward flavor and the cake may be more dense.
If you can’t find cultured buttermilk, try using kefir in its place, or plain runny yogurt (dairy or plant) thinned with a bit of milk (regular or plant) or water to the consistency of heavy cream.
Pan sizes: Double the recipe to make a 9-inch round layer cake or 9-inch square sheet cake, increasing the baking time as needed. Try tripling the recipe for a 9x12-inch sheet cake, increasing the baking time as needed (a double batch of frosting should be sufficient here). For cupcakes, fill 6-8 lined cupcake cups two-thirds of the way with batter and decrease the baking time as needed.Do-ahead: The cake can be made 1 or 2 days ahead and stored covered at room temperature. The cream cheese frosting can be made up to several days ahead, colored or not, and stored covered in the refrigerator. Bring to room temperature and stir well before using.Look for beet powder at health food stores with the herbs and spices, or order it here. (Extra beet powder is delicious with steamed milk and honey for a sweet beet latte!) If you don't have any on hand, simply top the cake with plain cream cheese frosting – it will still be delicious!I’ve given the measurements in grams which means that if you have a scale, you can weigh everything right into the bowl, zeroing out the scale between ingredients.Gluten-Free Dairy-Free Red Velvet CakeFor those who don't do dairy, swap the buttermilk in the recipe for DF yogurt thinned with plant milk and top the cake with vegan cream cheese frosting or paleo cream cheese frosting.Nutritional values are based on one of eight servings.