Preheat oven to 160℃ (140℃ fan-forced).
Grease and line a 24 cm round spring-form tin with baking paper.
Break 80g of chocolate into small pieces. Melt the butter over a low heat, then stir in the brown sugar and chocolate. Stir until the sugar has dissolved and the chocolate has melted.
Remove from heat, stir in the grated beetroot and set aside.
Whisk the eggs in a large mixing bowl, then pour in the beetroot mixture.
Sift the flour, baking powder, cocoa and salt into a separate bowl.
Fold the flour mixture into the beetroot mixture, then pour into the prepared tin.
Bake for 55-60 minutes until an inserted skewer comes out clean.
Cool in the tin for 5 minutes before turning out onto a rack to cool completely.
Before serving, grate the remaining 20 g of chocolate on top of the cake and sprinkle with rose petals. Serve with Greek yoghurt or a dollop of cream on the side.