Heat the milk with the cinnamon stick until boiling. Allow to simmer for 30 minutes without boiling. Mix the egg yolks with the sugar. Pour the warm milk on the egg yolks and heat again. Allow to warm to 82/83ºC while constantly stirring. Beware: if the temperature increasing above 83ºC, the egg yolks will solidify, which means the sauce will not be creamy. If this happens, sieve the sauce and allow to cool.