Preheat the oven to 180°C / 160°C fan / gas 4. Bring a medium saucepan half-filled with water to a gentle simmer. Place the chocolate and butter in a heatproof bowl and set it over the pan, stirring occasionally until melted. Remove from the heat and allow to cool slightly.
Whisk the sugar and eggs together in a separate bowl until pale and fluffy, then pour in the melted chocolate mixture. Sift the flour and cocoa powder over the top, grate in the beetroot and gently fold everything together until fully combined.
Line a shallow 20cm square tin with baking parchment. Spoon in the mixture and smooth the surface. Roughly break the walnut halves and scatter them across the top, pressing them lightly into the batter. Tear or finely chop the rosemary and sprinkle this over as well.
Bake for 15–20 minutes, until the top is cracked and set but the centre remains slightly gooey. Allow the brownies to cool in the tin for 25–30 minutes before cutting into squares.
These brownies will keep well in an airtight tin for up to one week.
