Crush the biscuits by putting them in a large, strong, plastic food bag and bashing with a rolling pin. Melt the butter with the syrup or honey in the microwave, then stir in the biscuits. Press onto the base and up the sides of a 12 x 36cm rectangular tin (or 23-25cm round flan tin) and chill while you make the filling.
Break up the two chocolate bars and put in a large bowl. Melt in the microwave for 2-3 mins on Medium, stirring halfway
through. Stir in the vanilla extract, then sift in the icing sugar. Whip the cream until it just holds its shape, then fold into the melted chocolate. Pour into the prepared tin and smooth the top. Chill for at least 2 hrs or for up to 2 days.
Just before serving, remove the tart from the tin and slide it onto a flat plate (loosen edges first with the tip of a small pointed knife). You will find it easier to remove the tart if you leave it at room temperature for 30 mins. Put a few tsps of crème fraîche along the centre of the tart (or around the edges if the tart is circular) and top each with a raspberry, then dust lightly with icing sugar. Cut into thin slices and serve with extra raspberries on the side.