Grease a box cake tin (approx. 2 1/2 l capacity; 30 cm long) and dust with flour. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
Cut the beetroot into small pieces. (Attention! They stain strongly, best to wear disposable gloves.) Puree beetroot and oil in a mixing bowl with a hand blender. Stir in the eggs one by one with the whisks of the mixer.
Mix 275 g flour, baking powder, 100 g cocoa, sugar, vanilla sugar and 1 pinch of salt. Add beetroot puree. Stir everything briefly to a smooth dough. Smooth in the mould. Bake in a hot oven for about 1 1⁄4 hours (the inside of the cake should still be slightly moist).
After approx. 1 hour, cover with aluminium foil if necessary.
Take the cake out of the oven. Let it cool in the mould for 20-30 minutes. Then carefully turn out of the mould, let it cool down and dust with approx. 2 tsp. cocoa.