Preheat the oven to 180 °C.
Melt the chocolate with the butter over very low heat. Stir occasionally. Remove the pan from the heat when everything is almost melted and let it cool down. Whisk the eggs with the vanilla sugar until fluffy.
Slowly add the pureed beets and the salt and mix well. Stir in the cooled chocolate-butter mixture.
Add the flour and cocoa.
Spread the mixture onto a baking sheet lined with parchment paper, about 1.5 to 3 cm thick. Sprinkle a layer of chopped nuts on top.
Bake for 10 minutes and let it cool completely before cutting the cake into squares.
Tips
Take the time: chocolate must slowly melt and bake! Don't have a beetroot at home? Or do you not like that at all (although you don't taste much of it in the brownie), take the same amount of apple sauce: that sweet just as well.