Preheat the oven t 160°C (fan). Grease a 20cm round, loose-bottomed cake tin with a little butter and line the base with baking paper.
Place the plain chocolate in a bowl and set over a pan of gently simmering water. Allow the chocolate to melt slowly until smooth, then set aside to cool.
Place the eggs, sugar and sunflower oil in a large mixing bowl and whisk together, using an electric hand whisk, for about 3 minutes until the mixture is smooth and creamy. Stir in vanilla extract, then sift over self-raising flour, bicarbonate of soda, baking powder and gently fold in, together with the ground almonds.
Using a pair of rubber gloves to protect your fingers from staining, whilst still warm, remove the skin from the beetroot. When it has cooled, grate, squeeze out some of the excess liquid. Fold the beetroot into the mixture with the cooled chocolate, until thoroughly mixed.
Pour the mixture into the prepared tin and bake for 35-40 minutes. Test the cake by inserting a skewer into the centre to see if it comes out clean. Cool for a little while, then remove from the tin and leave to cool completely on a wire rack.
Chop the 150g of chocolate for the ganache and place in a glass or heatproof bowl, add the remaining ingredients. Place in the microwave (800w) and heat for 30 seconds and jiggle the bowl to create a little movement then leave undisturbed for 5 minutes. Heat again for 20 seconds then stir gently but don't over stir as this will take the sheen away from the ganache. If all the chocolate has melted and it's a nice pouring consistency then pour over the cooled cake. Grate and chop the handful of chocolate and sprinkle on the cake once the ganache is completely cooled.