1. To make the custard, bring the milk, sugar, and the vanilla bean to a simmer in a medium saucepan over low heat. Remove the vanilla bean, and use the tip of a knife to scrape the beans back into the milk. Rinse the bean and save for another use or discard.
2. Whisk the egg yolks in a heatproof bowl. Gradually whisk in the hot milk. Rinse out the saucepan and pour the milk mixture back into the pan. Stirring constantly with a wooden spoon, cook over low heat until the custard is thick enough to coat the back of the spoon (an instant-read thermometer will read 185°F/85°C). Strain through a wire sieve into a bowl. Sprinkle the surface with 1 tsp sugar, to help prevent a skin from forming. Let cool.
3. Meanwhile, spread the sliced cake with raspberry jam and sandwich together in pairs. Cut into strips about 1¼ in (3cm) wide and arrange in the bottom of a 6-cup serving bowl.
4. Add the raspberries, pushing them down into the spaces between the cake strips. Sprinkle with the cookie crumbs, then drizzle with the sherry. Spoon the cooled custard the top. Cover and refrigerate at least 1 hour and up to 1 day.
5. Whip the cream until thickened but not stiff. Spread over the trifle and sprinkle with almonds. Serve chilled.
Leftovers can be served on fruit jelly, ice cream, or as an accompaniment to fruit salad.
Chocolate Banana Trifle
Substitute chocolate-nut spread for the raspberry jam, and spread on the cake slices. Substitute 1 or 2 sliced bananas for the raspberries. Add 7oz (200g) chopped bittersweet chocolate into the hot, strained custard, let stand until melted, then whisk until smooth.