1.Line a deep 19cm (8-inch) square cake pan with aluminium foil; grease top of foil.
2.Combine marshmallows, butter and the water in a medium saucepan over medium heat; cook, stirring, until marshmallows are melted. (Mixture will be bubbling gently while the marshmallows are dissolving).
3.Remove from heat, stir in chocolate and extract. Stir with a wooden spoon until chocolate is melted.
4.Stir in nuts; beat with a wooden spoon until mixture becomes sticky. Pour mixture into pan. Refrigerate for 4 hours or overnight before cutting into bars.
You can use pink or white marshmallows in this recipe and swap the walnuts with your choice of nut. Store the fudge in an airtight container in the refrigerator for up to 2 weeks.