1. Make custard: stir powder and sugar in a heavy-bottomed saucepan until blended.
2. Gradually whisk in milk, sherry and rum. Cook over medium heat, stirring frequently, until mixture comes to a boil.
3. Stir constantly until custard thickly coats a spoon, about 2 minutes.
4. Remove from heat and stir in vanilla extract.
5. Press piece of plastic wrap on surface to prevent a skin from forming and chill until cooled.
6. Cut jelly roll into 1/2 inch thick slices.
7. Place over sides and bottom of a slightly rounded glass bowl that will hold 3 L.
8. Cover cake with half of custard then layer of whipped cream.
9. Add some raspberries, then another layer of cake; cover with remaining custard.
10. Refrigerate, covered, for at least 6 hours or overnight.
11. Before serving, spoon whipped cream over trifle and drizzle with raspberry sauce.
12. Decorate with raspberries.