Line a 12-hole muffin tin with paper cases.
Put the chocolate, butter and golden syrup in a heatproof bowl set over a pan of barely simmering water, making sure the base does not touch the water. Stir until melted and smooth, then take off the heat. A microwave in 20-second bursts, stirring between each, works too.
Tip in the cornflakes and fold gently with a spoon until every flake is coated. Crush them as little as possible: whole flakes give better nests.
Spoon the mixture into the paper cases, pressing each one lightly with the back of the spoon so they hold together.
Chill in the fridge for 30 to 60 minutes until set firm. They keep in a sealed container in the fridge for up to 5 days.