Wear a pair of rubber gloves to stop your hands from staining, then top, tail and peel the beetroot - you’ll need about 400g flesh. Roughly chop and put into a large bowl. Add a splash of water, cover with cling film, then microwave on High for 12 mins or until tender.
Heat oven to 180C/160C fan/gas 4. While the beetroot cooks, butter then line a 20 x 30cm traybake or small roasting tin. Roughly chop the chocolate and cut the butter into cubes. Tip the cooked beetroot into a sieve, drain off any excess liquid, then put into a food processor or blender with the chocolate, butter and vanilla. Whizz until the mix is as smooth as you can get it. The chocolate and butter will melt as you do this.
Put the sugar and eggs into a large bowl, then beat using an electric hand whisk until thick, pale and foamy, about 2 mins. Spoon the beetroot mix into the bowl (it won’t look too pretty at this stage, but bear with me), then use a large metal spoon to fold it into the whisked eggs. Try to conserve as much air in the mixture as you can. Sift in the flour and cocoa powder, then gently fold these in to make a smooth batter.
Pour into the prepared tin and bake for 25 mins or until risen all over, with just the merest quiver under the centre of the crust when you shake the pan. Cool completely in the tin, then cut into squares.