Chop the chocolate into chunks and place in a non-stick saucepan with the condensed milk and the butter.
Heat very gently until melted and smooth.
Sieve the icing sugar and beat into the chocolate mixture until thoroughly combined.
Roughly chop the nuts (if using) and add these to the mixture.
Line a 20cm x 20cm square cake tin with baking paper and put the fudge mix into the tin.
Push the fudge mix into the corners and smooth the top.
Place in the fridge for at least 2 hours.
Tip out and cut into squares.