Melt the dark and milk chocolates, butter and golden syrup in a small heatproof bowl set over a saucepan of simmering water, stirring occasionally and making sure the bottom of the bowl does not touch the water. Stir in the biscuits, almonds, raisins and orange peel.
Line a 10 x 30 cm (4 x 12 inch) terrine dish or loaf tin with clingfilm. Spoon the mixture into the tin and smooth over the surface. Chill for at least 3 hours.
Turn the refrigerator cake out of the tin on to a board, then lightly dust with cocoa powder. Serve cut into slices.