Put a 55g/2oz piece of the white chocolate in the fridge, (this will make it easier to grate later). Break the remainder into small pieces.
Cream the egg yolks and caster sugar together in a large bowl. Whisk for about 2-3 minutes until the mixture is pale, thick, creamy and leaves a trail.
Pour the milk and cream into a small, heavy-based saucepan and bring to the boil. Pour on to the egg yolk mixture, whisking all the time. Pour back into the pan and place over a moderate heat. Stir the mixture with a wooden spoon until it starts to thicken and coats the back of the spoon.
Add the chocolate pieces and stir in until completely incorporated.
Remove the pan from the heat and allow to cool slightly. Cover the custard with a little icing sugar and piece of cling film to prevent a skin forming.
Place the swiss roll slices in a large glass bowl and sprinkle with the Kirsch. Scatter with fresh raspberries, reserving a few for decoration.
Pour the white chocolate custard over the swiss roll and leave to set in the fridge, preferably overnight.
To serve, decorate the trifle with the reserved raspberries. Take the piece of white chocolate from the fridge and finely grate over the trifle.
Finally, dust with a little icing sugar and place the mint sprigs on top.