2) Prepare the cake according to the directions on the package, but use 110ml of the sherry in place of 110ml of water. When baked, leave to cool in the pan for about 10 minutes, then invert onto a wire rack to cool completely. Cut the cake into large cubes, about 4cm wide.
3) Place the egg yolks, caster sugar and remaining sherry into the top of a double boiler or a bowl to fit over a large saucepan. Whisk until completely combined. Place the top pan or bowl over simmering water - do not let the water touch the bottom of the pan or bowl. With a wooden spoon, stir the mixture over a medium heat for about 8 minutes, until the custard is quite thick, about the consistency of mayonnaise. Allow to cool.
4) Whip the cream with the icing sugar. Fold 1/2 of the whipped cream into the cooled custard.
To assemble the trifle:
1) In the bottom of a trifle bowl, layer 1/3 of the cake cubes, 1/3 of the raspberries and 1/3 of the custard. Continue layering, ending with custard.
2) Take the remaining whipped cream and completely cover the top of the custard. Refrigerate the trifle for at least 4 hours before serving.
3) Serve with additional whipped cream, fresh raspberries and a sprig of mint, if desired.