01 - Heat milk in a saucepan until just simmering. Whisk egg yolks, sugar, cornstarch, vanilla, and salt in a bowl. Gradually add hot milk to egg mixture, whisking constantly. Return to saucepan and cook over medium heat, stirring for 3–5 minutes until thickened. Remove from heat, cover with plastic wrap touching surface, and cool to room temperature.
02 - Beat cold heavy cream with powdered sugar in a chilled bowl until medium peaks form.
03 - Place half the sponge slices in a trifle dish or individual glasses. Spread half the custard over sponge, then a layer of whipped cream. Repeat with remaining sponge, custard, and whipped cream.
04 - Refrigerate for at least 1 hour to allow flavors to meld.
05 - Before serving, top with fresh berries, chocolate shavings, or chopped nuts as desired.