Melt plain chocolate, golden syrup and Frangelico (if using) in a heatproof bowl set over a pan of simmering water – do not stir or the mixture will seize into a solid mass. Take off heat and set aside. Next, melt white chocolate as above; take off heat; set aside. Allow both chocolate bowls to cool until just warm, about 15min.
Meanwhile, line base and sides of a 20.5cm (8in) round cake tin, at least 9cm (3½in) deep, with baking parchment – use a little golden syrup to help stick the parchment. Roughly crumble brandy snaps into base of tin.
In a large bowl, whip cream and icing sugar until mixture just holds its shape. Spoon roughly ⅓ of cream mixture into white chocolate bowl. Add hazelnuts; fold to combine.
Next, fold melted plain chocolate mixture into remaining cream. Spoon into prepared tin and smooth to level, trying not to disturb brandy snaps.
Sprinkle on about ⅓ of the raspberries, then top with the white chocolate mixture, smoothing to level. Arrange remaining raspberries on top. Chill for at least 1hr.
