Grease a 20 x 24.5cm tin with butter and line it with a single sheet of greased parchment allowing a few centimetres of overhang (to use as handles when removing the fudge).
Place the sugar, butter, milk, chocolate and golden syrup into a large heavy based saucepan.
Heat gently and stir until all the sugar has dissolved (you will hear it scratching against the bottom of the pan with your wooden spoon if it hasn't).
Bring the mixture to the boil and then boil gently for 10 minutes or until it reaches the “soft ball” stage of 116ºC on your food thermometer (has to be suitable for sugar).
Once this stage is reached remove the pan from the heat and set aside to cool for 5 minutes.
Add the biscuit pieces and the marshmallows and beat until the mixture starts to look thick and almost grainy.
Pour the fudge into the greased tin and even out the surface with a plastic spatula.
Scatter mini marshmallows over the top pressing down with your hand.
Then scatter the plain and white chocolate chips over the top and allow to cool.
Once cool remove the fudge from the tin using the parchment “handles” and cut into even sized pieces. (I made 30 but you could make more by cutting smaller).
Then store in an airtight container where it should keep for at least 2 weeks.