Melt the white chocolate in a bowl suspended over boiling water. Once melted, remove from the heat and allow to cool slightly.
Whisk the crème fraîche and double cream together until smooth and then add the slightly cooled chocolate.
Divide the mousse mixture between 2 small serving dishes and allow to cool to room temperature.
Meanwhile, to make the chocolate buttons, melt the chocolate in a dish over a pan of simmering water. Put several spoonfuls of melted chocolate onto greaseproof paper and allow them to spread out into button shapes. Add a few sprinkles to each button. Place in the fridge for 20 minutes to chill.
Once chilled, add the buttons to the mousse to decorate then place the desserts in the fridge to chill for about 90 minutes before serving.