Bring the single cream or milk slowly to the boil with the cinnamon. Remove the pan from the heat, cover, and leave to infuse for 20 minutes. Whisk the egg yolks and sugar until thick and pale. Gently reheat the cream, pour a little into the yolks, then pour this mixture into the pan and return it to a very low heat. Stir for several minutes until the cream is thick enough for your finger to draw a line on the spoon. Do not let the mixture boil or it will curdle. Should that happen, lift the pan from the heat at once, add a spoonful of cold milk or cream and beat hard; this usually remedies it.
Whip the double cream lightly and fold it into the custard. Freeze in an ice cream machine following the manufacturer’s instructions. If you do not eat the ice cream straight away, put it in a plastic container in the freezer and transfer it to the refrigerator for 30 minutes before serving.