Preheat your oven to 175°C. Grease a 9-inch cake tin and line the bottom with baking parchment.
In a bowl mix together the flours with the baking powder and in another bowl, combine the cocoa with the hot water to form a smooth paste.
Using a stand mixer or a hand-held mixer, beat together the eggs, salt and sugar on a medium speed till the mixture is thick, foamy and creamy (about 3 minutes).
Tip the flours into the egg mixture followed by the vanilla extract and the coconut oil. Mix until just combined.
Now, gently mix in the cocoa paste so that it is well combined.
Finally, fold in the grated beetroot, juice and all.
Pour into the prepared tin and bake for 20 minutes or until you get a clean toothpick. Let the cake cool completely on a wire rack before topping with the ganache.
To make the ganache, melt the chocolate with the heavy cream in a heatproof bowl set over (not in) a pan of barely simmering water. Stir vigorously till you get a smooth and glossy ganache. Let it cool.
Using a palette knife, spread the ganache generously to coat the top and sides of the cake.
Chill the cake in the fridge for a couple of hours to let the ganache set. Slice and serve.