Preheat the oven to 170C/325F/ gas mark 3. Lightly
grease a baking tin that's roughly 20cm x 30cm in size and at least 2cm deep.
Line the bottom with greaseproof paper and butter the paper,
too.

Put
the cubed butter and chocolate into a heatproof bowl. Place this on an oven tray
lined with a baking sheet, and put in oven to warm up. After a few minutes,
remove, stir, then return to the oven to melt completely. (Alternatively, melt
the chocolate and butter in the conventional manner, in a bowl held over a pan
of barely simmering water.)

In
another bowl, whisk the sugar with the eggs until smooth and creamy. Stir in the
chocolate mixture until well combined. Sift in the flour, stir, fold in the
walnuts (if using) and beetroot. Pour into the prepared
tin.

Bake for 20-25 minutes, until a knife or skewer comes
out with a few moist crumbs clinging to it - be careful not to overcook the
brownies. Remove from the oven, then stand the tray on a wire rack until cool
enough to cut into squares.