A cake with beets per ingredient, which is not rich in sugar? And maybe a cocoa cake? 
Here is a low-calorie recipe that uses the sweetness of beets to reduce some sugar and still have a cocoa cake. Ideal for those who cannot eat vegetables a lot, it is rich in monounsaturated and medium-chain healthy fats and rich in vitamins.
In short, it is a health cake. The only thing: it should be eaten completely cold, so as not to feel the “vegetable” flavor. Ideal for breakfast.

Here is the recipe for the cocoa and beetroot cake:
for 8 people 
300 gr of previously boiled beets
75 gr unsweetened cocoa

150 gr stone-ground wholemeal flour
100 gr sugar (coconut palm sugar ideal, for those who want to cut calories well erythritol)
3 tablespoons of coconut oil (alternatively extra virgin olive oil)
a pinch of salt
300 gr of egg whites
3 eggs
vanilla flavor
baking powder, one sachet

Finely chop the boiled beets, until you get a puree, and mix it with the cocoa, salt, vanilla flavor.
Whip the three eggs with the sugar and coconut oil. Add the cocoa and beetroot mixture and finally the sifted flour with yeast.
Finally, the 300 grams of egg whites whipped until stiff. Pour into a greased pan and cook at 170 degrees for about 35 minutes.