I gently heat the milk until steamy. Meanwhile, whisk egg yolks, sugar, and cornstarch smooth. I temper the yolks with warm milk, then cook and whisk until thick, stir in vanilla and butter, then chill.
I brush the cake layers with sherry and spread the jam over them. I toss sliced strawberries and raspberries with sugar and a splash of sherry so they get juicy and vibrant.
I place a third of the cake cubes in the bowl, top with a third of the fruit, then a third of the chilled custard, and a third of the whipped cream. Then I repeat the sequence two more times. Chill before serving for best results