Lightly grease a 20cm square shallow baking tin
Put the chocolate, condensed milk and butter into a small saucepan and melt gently over a low heat, stirring occasionally, until smooth and silky. Sift in the icing sugar and mix thoroughly.
Press the mixture into the prepared tin and smooth over the top with the back of a spoon. Cover with clingfilm and leave to set in the fridge for one hour.
Once set, turn out the fudge onto a chopping board and cut into squares or rectangles. You can store this in the cupboard / Pantry in an airtight container for a week.
TIP: If you are looking for some variety, add chopped nuts into the mixture after you have mixed in the icing sugar.