Heat oven to 180C/160C fan/ gas 4 and line a muffin tin with eight paper cases. Melt the butter in a small pan, then leave to cool for 5 mins. Tip the sugar, cocoa and flour into a food processor and whizz to get rid of any little clusters of sugar that are sticking together. Tip in the melted butter and eggs, and blitz to make a smooth batter.
Scoop a little of the mixture into each muffin case, then lightly press a white chocolate truffle in the centre. Spoon over the remaining mixture, making sure that the truffles are covered. Bake for 15-20 mins until the mixture has firmed up. Leave to cool.
To decorate the muffins, tip the butter and sugar into a large bowl, add the vanilla and beat with an electric hand whisk until really smooth. Thinly spread a little of the vanilla mixture over the top of the cakes to cover them, taking the icing down the sides to meet the paper case. Beat the cocoa powder into the rest of the icing, then put in a piping bag fitted with a large star nozzle and pipe hearts onto the top of each cake. Add a chocolate heart to decorate for the final flourish.