Melt the chocolate and butter together in a pan over very low heat. Stir until smooth, then allow to cool.
Puree the beetroot with the orange liqueur - I used a stick blender.
Whisk the eggs and sugar together until the mixture is thick and has doubled in volume.
Fold in the chocolate mixture, followed by the beetroot trying not to stir all of the air out of the egg mixture.
Sieve in the flour and cocoa and fold in until just combined.
Pour into a 9" (23 cm) sq silicone cake mould and bake in the middle of the oven at 180℃ (350℉, Gas 4) for about 30 minutes when the top should be firm, but the middle still a bit squidgy.
Allow to cool in the mould, then cut into 16 squares.
If you don't have orange chocolate, you can swap it for plain and use the grated zest of an orange instead.Likewise, you can swap the orange liqueur for orange juice, or just leave it out altogether.Please note: calories and other nutritional information are per serving. They're approximate and will depend on serving size and exact ingredients used.