1 For ease of shaping, line a flat, square tin with an edge (approx. 20 x 20 cm) with aluminium foil.
2 Put the sugar, condensed milk, butter and marshmallow spread in a small pot with a pinch of salt. Cook at a rolling boil for around 5 minutes, stirring. Remove from the heat.
3 Gradually stir the two types of chocolate through the mixture and allow to melt.
4 When the mixture is smooth and glossy, pour it into the prepared tin. Smooth the top flat and chill in the refrigerator for around 2 hours, longer if needed, until firm.
5 Take the fudge out of the tin and remove the foil. Cut into small squares using a knife dipped in hot water.