Prepare the fruit: In a bowl, toss the berries with 2 tablespoons sugar and let macerate for 15 minutes.
Make the custard: In a medium saucepan, heat milk over medium heat until just simmering. In a separate bowl, whisk egg yolks, 1/2 cup sugar, and cornstarch until smooth. Slowly pour hot milk into egg mixture while whisking, then return to saucepan. Cook over medium heat, stirring constantly, until thickened (about 5 minutes). Remove from heat, stir in 1 teaspoon vanilla, and let cool slightly.
Whip the cream: In a chilled bowl, beat heavy cream, powdered sugar, and 1/2 teaspoon vanilla until soft peaks form.
Assemble the trifle: Place a layer of cake cubes in the bottom of a trifle dish or glass bowl. Drizzle with half of the sherry or juice. Spoon half of the macerated berries over the cake. Pour half of the custard over the berries. Repeat layers: remaining cake, sherry, berries, and custard.
Top with whipped cream, spreading evenly. Cover and refrigerate for at least 2 hours or overnight.
Before serving, garnish with fresh mint if desired.
Tips
For a non-alcoholic version, use fruit juice like orange or apple juice instead of sherry.
Trifle can be made a day ahead and refrigerated; the flavors meld beautifully.
Use a clear glass dish to show off the beautiful layers.