Preheat the oven to Gas Mark 4, 180°C, fan 160°C. Put the butter, chocolate and sugar in a large saucepan. Cook over a low heat until the chocolate has melted. Add the vanilla extract and leave to cool. Whizz the beetroot in a food processor until chopped. Add the chocolate mixture and whizz together until smooth, scraping down the sides occasionally. Add the eggs, flour and cocoa powder and pulse until well combined. Lightly grease a 20cm x 30cm (8in x 12in) baking tin and line with with nonstick baking paper. Spoon the mixture into the tin and smooth over surface with the back of a spoon. Bake for 25 minutes. Cool in the tin, cut into slices and serve warm or cold. Freezing and defrosting guidelines Bake according to the recipe but stop before adding decoration or icing if called for. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months. Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven. If you wish to add icing or decoration, add them after the baked goods have fully defrosted and are at room temperature.