Heat together the butter, condensed milk, syrup and sugar in a heavy-based saucepan. Stirring occasionally, bring to the boil.
Boil steadily for 7-8 minutes or until thickened, stirring from time to time to prevent the mixture from burning. Remove from the heat and stir in the crushed biscuits.
Tip the mixture into a 25 x 7 cm greased shallow baking tray.
Decorating
Whilst the fudge is still warm, scatter on the marshmallows, gently pressing them into the surface.
Melt the chocolate in a bowl set over a saucepan of barely simmering water, then drizzle over the marshmallow topping. Before the chocolate sets add some delicious white chocolate sprinkles.
Chill for a minimum of 2 hours or until set then cut into squares.