Preheat the oven to 180oC/Gas Mark 4. Grease and line a 900g (2lb) round cake tin, or use a round silicone mould.
Boil the sweet potatoes then, once soft, purée in a food processor and measure out 125g (4oz) and set aside.
Add all the other cake ingredients to a food processor, including the puréed sweet potatoes, then blitz until you have a lovely smooth mixture.
Pour the mixture into the cake tin and place in the oven for 40-45 minutes.
While the cake is cooking, make the icing by blitzing the coconut yogurt, beetroot and vanilla pod with a splash of maple syrup.
When the cake is ready it should feel firm and a toothpick inserted into the centre of the cake should come out clean.
Once out of the oven, let the cake cool a little, then turn it out of the cake tin and let it cool completely on a wire rack.