1. Step 1

    Heat together the butter, condensed milk, syrup and sugar in a heavy-based saucepan. Stirring occasionally, bring to the boil.

  2. Step 2

    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.

  3. Step 3

    Tip the mixture into a 25 x17cm (10x7in) greased shallow baking tray. Whilst the fudge is still warm, scatter on the marshmallows, gently pressing them into the surface.

  4. Step 4

    Melt the chocolate in a bowl set over a saucepan of barely simmering water, then drizzle over the marshmallow topping. Chill for a minimum of 2 hours or until set then cut into squares.