Whisk together egg yolks, granulated sugar, and cornstarch in a bowl. Heat milk in a saucepan until it simmers, then slowly whisk half of it into the egg mixture to temper it.

Pour the tempered egg mixture back into the saucepan with the remaining milk and cook over medium heat, stirring constantly, until the custard thickens. Remove from heat, stir in vanilla extract, and let it cool completely.

Arrange half of the cubed pound cake at the bottom of a large trifle bowl. Drizzle with half of the Grand Marnier or orange juice, if using.

Layer half of the fresh mixed berries over the cake, then spread half of the cooled custard evenly over the berries.

Repeat the cake, Grand Marnier/orange juice, berry, and custard layers. Ensure the final custard layer is smooth and even.

In a separate bowl, whip heavy cream with powdered sugar and vanilla extract until stiff peaks form. Spread the whipped cream over the top custard layer.

Garnish with extra berries if desired. Refrigerate the trifle for at least 4 hours, or preferably overnight, before serving to allow the flavors to meld and the cake to soften.