Preheat your oven to 180°C/160°C fan/gas mark 4. Grease and line a 20cm square cake tin.
Prepare your beetroot by trimming off any leaves, then peel and cut into quarters. Add to a microwaveable bowl with a splash of water and cover with clingfilm. Cook on high power for 10-12 minutes until cooked through. Leave until cool enough to handle and then grate – you should be left with approximately 250g cooked beetroot. Set aside.
Place a medium saucepan of water on the stove and fill a third of the way with water. Bring to a simmer and place a large heatproof bowl on top, adding the chocolate and butter. Making sure the bowl isn’t touching the water, stir the chocolate and butter until melted, then remove.
Whisk the eggs and sugar together until thick and creamy. Pour in the melted chocolate, then fold until combined. Add the flour and fold until combined, then fold through the grated beetroot.
Pour the brownie mix into the lined tin. Spread out so the surface is even, then bake for 55 minutes to 1 hour until only a few crumbs remain when you poke with a skewer or cocktail stick.
Remove the brownies from the oven and place on a wire rack. Leave until entirely cool, then lift out of the tin. Dust with the cocoa powder and cut into 20 squares.