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For the base, melt the chocolate with the butter and golden syrup in a bowl over barely simmer watering or on a low heat in the microwave.
Put the biscuits in a bag and crush with a rolling pin until they reach a crumb consistency. Combine the two and spread over the base of a 23cm springform tin. Refrigerate for about an hour to set.
For the mousse, melt the remaining plain chocolate and the milk chocolate together in the same way and allow to cool until lukewarm. Using an electric beater, whisk the eggs whites in one bowl to stiff peaks. Softly whip the whipping cream in another bowl until thickened but not as far as soft peaks.
Stir the egg yolks into the chocolate, then beat the cream into the mixture - a balloon whisk is good for this. Work quickly if the chocolate looks like it might be becoming grainy. Gently fold in the eggs whites with a whisk, spread the mixture over the biscuit base and return to the fridge overnight.
Release from the tin and decorate with concentric circles of raspberries and serve.