Cut sponge cake into small cubes and arrange in the base of individual martini or serving glasses. Heat raspberries and sugar in a saucepan until the raspberries have just thawed and spoon over sponge cake. Place yolks, sugar and marsala in a large metal bowl over a saucepan of simmering water and whisk until thick and fluffy, taking care not to boil the mixture as it will curdle. (With experience this can be done over direct heat, by holding the bowl with an oven mitt and working quickly). Pour the zabaglione over the raspberries and serve immediately.
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