Slice the cake and line the bottom of the trifle dish. Two layers of cake should be enough. You will use most of the cake. Now, eat the cake that doesn't fit into the trifle bowl before anyone else gets to it. Drizzle the sherry evenly over the cake. If you like Harvey's Bristol Cream, pour yourself a glass.

In a heat proof dish (I use a large Pyrex measuring cup), pour the 1 cup of boiling water over the Jello powder. Once it is dissolved completely, add the frozen berries and keep stirring them around until they are starting to just set. Pour the berry mixture over the cake and even it out. Put the trifle dish in the fridge for at least 2 hours.

In a large glass measuring bowl, mix the custard powder, sugar and whisk in the milk. Cook in the microwave on high for 6-8 minutes. Stir every two minutes but do not walk away from the microwave because when it is nearly done it goes quickly and it will boil over. It is a supreme pain in the rear to clean up custard in the microwave. When the custard comes to a boil it will be nice and thick and it is done.

Put a piece of plastic wrap right on the surface of the custard and leave it on the counter to cool for a half hour. While it cools, take the trifle dish out of the fridge so that it can get the chill off. If you put burning hot custard into a very cold bowl you will most assuredly crack your dish - I've seen it happen. Put the trifle dish in the fridge with the top lightly covered (not on the surface of the custard as before).

When whole dish is nice and cold, cover with freshly whipped cream and serve.