Line an 8x8-inch tin with baking paper, leaving a little hanging over the edge for easy removal later.
Break the chocolate into pieces and add it to a large heatproof bowl with the condensed milk.
Place the bowl over a saucepan of gently simmering water on the stove to create a double boiler. Make sure the bottom of the bowl isn't touching the water.
Keep the heat low and stir the mixture gently until the chocolate has melted. It should be thick and glossy.
Transfer the mixture to your lined tin and press it down into place.
Place the M&Ms on top of the fudge while it's still wet, in 6 rows of 5.
Put the fudge in the fridge for 1-2 hours until firm, then cut into squares and enjoy.