Line an 8x8-inch tin with baking paper and set aside.
Break the chocolate into pieces and place it into a heatproof bowl with the condensed milk.
Place the bowl over a saucepan of gently simmering water to make a double boiler. Stir gently until the chocolate has melted. Alternatively, put it in the microwave in very short bursts, stirring each time until melted.
Remove the pan from the heat and stir in half of the Smarties. Quickly transfer the mixture to your lined tin and press it down into place.
While the fudge is still wet, add the rest of the Smarties on top and gently press them in.
Put the fudge in the fridge for 1-2 hours until firm. Remove from the tin and cut into squares.
Video
Notes
Store in an airtight container for up to 2 weeks.
For a cleaner cut, warm your knife before cutting. To do this, dip the knife into boiling water then dry before using.