Spread the sponge fingers with the blueberry jam and put in a glass bowl. Sprinkle over the sherry or white wine and half of the blueberries.
Mix a little of the milk with the cornflour until smooth. Stir into the rest of the milk. Pour into a saucepan and bring to the boil, stirring continuously as the milk thickens. When it is boiling and smooth, remove the pan from the heat.
Whisk the egg yolks and sugar in a bowl until light and creamy. Add the milk slowly, whisking all the time. Mix well, pour over the blueberries and sponge fingers, then sprinkle the rest of the blueberries over the top. Leave to cool, then chill, preferably overnight.
When ready to serve, whip the cream until softly peaking and spread it over the trifle. Sprinkle the grated chocolate over the top.