The day before, preheat the oven to fan oven 140C/conventional 160C/gas 3. Tip the raspberries over the base of a large deep, ovenproof dish. Press down lightly with the back of a spoon to release some of the berry juices.
Cover the raspberries with the slices of cake, then spoon over the Drambuie and orange juice. Leave to soak for 10 minutes.
Whisk together 600ml of the double cream with the eggs, sugar, orange zest and vanilla extract or essence. Spread the jam over the sponge to cover completely then slowly pour on the custard mixture.
Sit the dish in a roasting tin with enough hot water to come at least 5cm up the sides. Bake for 1 hour-1 hour 10 minutes, or longer depending on the height and width of the dish. The custard should be just set with a little wobble left in it. Leave to cool then cover with cling film and keep overnight in the fridge.
To serve, whip the remaining cream until it just holds its shape, then spoon it over the custard. Scatter over the raspberries and toasted shortbread and finish with a dusting of icing sugar.