Pour some boiling water into a pan and place on the lowest heat setting on hob, place a large bowl over the pan (careful not to touch the water) and add the white chocolate so that it melts.
Once melted whisk in the egg yolks and then the vanilla sugar until thoroughly dissolved. Set aside to cool.
Puree the strawberries in a mini blender or use a stick blender.
Beat the puree into the chocolate mixture.
Pour a small amount of double cream into a small pan (about 40 mls) heat very gently and sprinkle on the gelatine, take off the heat and stir until dissolved. Return to heat if needed.
Once gelatine is dissolved whisk this into the chocolate mixture and put in fridge to cool.
Meanwhile whip the eggs whites into stiff peaks.
Then whip the double cream into very soft peaks (over whipping will make it difficult to incorporate into the mousse).
Take the chocolate mixture out of the fridge and beat in large spoonful of the whisked egg white, then fold in the rest getting rid of any lumps but don’t over work.
Next fold in the softly whipped double cream until fully mixed in (if you did over whip then you might need to whisk in).
Divide between 4 sundae dishes or goblets and chill for at least 30 minutes before piping on rosettes of whipped double cream and decorate with sliced strawberries
Chill for at least another 1 ½ hours and serve.