Heat the milk, cream and vanilla in a heavy bottomed medium saucepan, stirring frequently, until it just begins to simmer and steam rises from the surface.
Mix the sugar and cornstarch together with a fork. In a large mixing bowl, whisk together the egg yolks and the sugar mixture until light and smooth.
Remove the warmed milk from the stove and while whisking the egg mix constantly, dribble a few drops at a time into the yolks. Adding the hot liquid very, very slowly in the beginning will temper the eggs, allowing them to warm gradually so that they don't curdle, or worse, scramble! Once you have dribbled in a good amount you can increase the amount of liquid you add at a time, whisking continuously until all the milk has been added.
Pour the contents of the mixer bowl into the saucepan and heat over medium, stirring constantly. Keep stirring, ensuring that the milk mixture does not stick to the bottom of the pan, until thickened. This should take you anywhere from 2-6 minutes, until the custard is thick and smooth. Pour into a shallow bowl and cover the top with plastic wrap so that the plastic is touching the surface of the custard. This will prevent a skin from forming on top. Cool in the refrigerator until chilled.
For the Cake
Cut the cake into large slices, brush them with the cream sherry (or juice) and then spread raspberry jam over them.
For the Fruit
In a medium bowl, mix the strawberries, raspberries, (sherry) and sugar and stir. Keep in the refrigerator until ready to assemble the trifle.
For the Whipped Cream
Combine the fresh cream and the powdered sugar in a bowl. Beat on high for about 2 minutes, or until stiff peaks form. Keep chilled in the refrigerator until ready to assemble.
Assemble the Trifle
Place 1/3 of the cake cubes in the bottom of the trifle dish (or large glass), jam side up. Top the cake cubes with 1/3 of the fruit, followed by 1/3 of the custard and finally with 1/3 of the whipped cream. Repeat layers two more times. Decorate the top with fresh fruit. Keep chilled until ready to serve.