In a heavy-based saucepan, heat together the butter, condensed milk, syrup and sugar. Stirring constantly, bring to the boil.
Boil steadily for 7-8minutes until thickened, stirring from time to time to stop the mixture burning. Remove from the heat and stir in the crushed biscuits. Pour the mixture into a greased shallow baking tray. While the fudge is still warm, press on the marshmallows, spacing them evenly.
Melt the chocolate in a bowl set over a pan of warm water or in the microwave and drizzle over the marshmallow and fudge mixture. Chill until set, then cut into squares.
*Inspired by Gemma S. featured in the Realfood Cookbook