Put both the chocolates, butter and the golden syrup into a medium saucepan and set over a medium-low heat. Stirring frequently, allow everything to melt together into a smooth, chocolatey pool.
Put all the remaining ingredients into a large mixing bowl.
Pour the chocolate mixture into the mixing bowl and stir so that everything is evenly mixed and coated in the chocolate.
Scoop into the prepared cake tin, press down to level and refrigerate for an hour or until set solid. Cut into the desired number and sized pieces.