Preheat oven to 180°C and grease a 30x20cm traybake tin.
Mix flour, cocoa powder, and baking powder in a bowl.
Purée cooked beetroot in a food processor and combine with vinegar and buttermilk.
Beat butter and sugar until light, then beat in eggs and half of the dry and wet mixtures.
Add remaining ingredients, pour into tin, and bake for 40-50 minutes before cooling.
Beat cream cheese, butter, icing sugar, and vanilla to make the frosting and spread on the cooled cake.
Thinly slice raw beetroot and cut into 12 heart shapes.
Boil 200g sugar in 200ml water, simmer hearts for 10 minutes until translucent, and dry on paper towels.
Slice the cake into 12 squares and decorate each with a beetroot heart.