In a small saucepan, heat the milk for the custard over low-medium heat. Stir frequently until the milk begins to simmer and steam starts to rise. 

In a large mixing bowl, whisk together the egg yolks, sugar, and cornstarch until smooth.

Remove the milk from the stove. Whisk the egg mixture constantly as you slowly add the milk a few drops at a time. If you add the milk too quickly, the heat will curdle the eggs. Whisk continuously until all the milk has been added.

Pour the contents into the saucepan and heat over medium, whisking constantly, until it boils. Continue whisking while it boils for about 1-2 minutes or until the mixture thickens.

Remove from heat and whisk in the vanilla. Allow custard to sit for 5 minutes.

Once the custard has rested for 5 minutes, whisk in the butter. 

Pour custard into a shallow bowl and cover with plastic wrap so that the wrap touches the surface of the custard. This will prevent the custard from forming a skin. 

Refrigerate until chilled.

Cut the cake in half horizontally like a hamburger bun and brush the cut sides with the cream sherry.

Spread raspberry jam over the sherry. Cut the halves into small squares.

In a medium bowl, combine the fresh strawberries, fresh raspberries, sherry, and sugar. Stir to coat and allow to macerate. 

Refrigerate the fruit until ready to assemble.

In a large bowl, combine the heavy whipping cream and powdered sugar. This is easiest in a standing mixer or with a hand mixer. 

Beat on high for 2 minutes or until stiff peaks form. 

Refrigerate until ready to assemble.

To assemble, use a trifle dish. Place 1/3 of the cake cubes jam side up in the bottom of the dish. Top with 1/3 of the fruit mixture, 1/3 of the custard, and 1/3 of the whipped cream - in that order. Repeat all the layers twice more. 

Decorate the top with fresh fruit. 

Refrigerate until serving.