Melt the butter, milk chocolate, dark chocolate and golden syrup together and stir thoroughly. (See recipe tips above regarding melting.)
Allow the chocolate to cool slightly and then stir in the Maltesers and mini marshmallows.
Line a square (about 20cm x 20cm) baking tin with baking paper and press the mixture firmly in.
Melt the white chocolate and pour all over the top. Press in the remaining Maltesers.
Allow to set in the fridge for at least a couple of hours.
Turn out of the tin and cut into small chunks using a very sharp knife.
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