Preheat oven: 180°C / 160°C fan / 350 °F / Gas Mark 4
Peel and coarsely grate or shred your beetroot.
Place the grated beetroot in a clean tea towel or muslin cloth and squeeze firmly over a bowl. Reserve 2 - 3 tablespoons of beetroot juice for the drizzle and set the squeezed beetroot aside.
Place the chocolate and butter in a microwave safe bowl. Microwave on high for 1 - 2 minutes in 30 second bursts, stirring after each one, until melted.
Stir in the vanilla extract until smooth, then set aside to cool slightly.
In a large bowl, whisk the eggs and sugar together with an electric mixer until thick, pale and doubled in volume. This takes approximately 5 - 8 minutes, depending on your mixer. When you lift the whisk, the mixture falling from it should leave a trail on the surface for a second or two.
Pour the cooled chocolate mixture into the whisked eggs and sugar. Gently fold together with a silicone spatula, taking care not to knock out the air.
Fold in the grated beetroot in a figure of 8 motion. Take your time, you don’t want to knock the air out.
Sift the flour, cocoa powder, baking powder, salt and optional espresso powder over the mixture. Fold gently until just combined.
Pour mixture into the prepared tin, gently easing the mixture into the corners of the tin and levelling with a spatula.
Bake for 25 minutes. The brownies should have a thin, papery crust on top and the centre should still wobble slightly when you gently move the tin.
A toothpick inserted into the centre should come out with a smear of brownie and a few moist crumbs attached. If there is still wet batter on it, return the brownies to the oven for another five minutes.
Once baked, leave in the tin until completely cold - if you can! Then use the overhanging parchment paper to lift them out.