Chop the beetroot. Use a hand-held blender or small food processor to make about ½ cup of puree.
Preheat oven to 180°C. Line a baking tray or gastronome with non-stick baking paper. Melt the chocolate and butter in a heatproof bowl over a saucepan of simmering water, stirring often until smooth. Allow to cool slightly.
Whisk in the eggs and vanilla topping. Add the beetroot puree. Whisk until well combined.
Sift the flour and cocoa into a large bowl. Add the sugar and chocolate mixture. Whisk until just combined. Stir in the walnuts. Pour into the baking pan, and smooth the surface. Bake for 20 minutes or until the top is firm and crumbs cling to a skewer when inserted into the centre. Cool completely in the pan before slicing.
Serve one portion with double cream and garnish with icing sugar and chocolate topping.