With an electric whisk, whip together the mascarpone and custard in a bowl. Cover and chill until ready to use. Roughly mash most of the raspberries with the elderflower cordial.
Cut up the sponge cake and use to line the bottom of four glasses. Drizzle each one with a little kirsch, and spoon over half of the raspberry mixture. Cover with half of the whipped mascarpone and custard, then repeat the layers with the remaining berries and mascarpone and custard. Decorate with the rest of the raspberries and chill for at least 30 mins before serving.