Step 1: Prepare the Cake Layer. Arrange half of the pound cake cubes evenly at the bottom of a large (3-4 quart) trifle bowl. Drizzle half of the sherry or fruit juice over the cake cubes, allowing them to soak slightly.
Step 2: Make the Custard. In a medium saucepan, whisk together the egg yolks, granulated sugar, and cornstarch until smooth. Gradually whisk in the whole milk. Heat over medium heat, stirring constantly with a whisk, until the custard thickens and comes to a gentle boil. This typically takes about 8-10 minutes. Remove from heat and stir in 1 teaspoon of vanilla extract. Pour half of the hot custard over the cake layer in the trifle bowl. Cover with plastic wrap, pressing it directly onto the surface of the custard to prevent a skin from forming. Let cool slightly.
Step 3: Add Fruit and More Cake. Once the first custard layer has cooled for about 15-20 minutes, sprinkle half of the fresh mixed berries over the custard. Arrange the remaining pound cake cubes on top of the berries, then drizzle with the remaining sherry or fruit juice. Pour the rest of the custard over this second cake layer, pressing plastic wrap onto the surface again. Refrigerate for at least 1-2 hours to allow the custard to set.
Step 4: Prepare the Whipped Cream. In a large mixing bowl, using an electric mixer, whip the very cold heavy cream with powdered sugar and 1/2 teaspoon vanilla extract on high speed until stiff peaks form.
Step 5: Assemble and Chill. Once the trifle base is well chilled and the custard is set, spread the whipped cream evenly over the top layer of custard. Garnish with the remaining fresh mixed berries. Refrigerate for at least 4 hours, or preferably overnight, for the best flavor and texture development. Serve chilled.