Prepare the custard: In a medium saucepan, heat the milk and sugar over medium heat, stirring occasionally until the sugar is dissolved. In a separate bowl, whisk the egg yolks and cornstarch together. Once the milk mixture is warm (but not boiling), slowly pour it into the egg mixture while whisking constantly to prevent curdling. Return the mixture to the saucepan and cook over medium heat, stirring constantly, until it thickens. Remove from heat, stir in the vanilla extract, and allow it to cool slightly.

1. Whip the cream: In a mixing bowl, beat the heavy whipping cream, powdered sugar, and vanilla extract with an electric mixer until stiff peaks form. Set aside.
2. Assemble the trifles: In individual serving cups or glasses, start by layering pieces of sponge cake at the bottom. If desired, drizzle a bit of fruit juice or dessert liqueur over the cake to add moisture and flavor. Next, add a layer of custard, followed by a layer of fresh fruit. Repeat the layers until you reach the top of the glass, finishing with a generous dollop of whipped cream.
3. Chill and serve: Refrigerate the assembled mini trifles for at least 1-2 hours to allow the flavors to meld together. Just before serving, top with extra fresh fruit for garnish.