Put the milk, sugar and piece of cinnamon stick in a heavy bottomed saucepan, bring to the boil, remove from the heat and add the bicarbonate of soda mix. It will foam up so be careful, stir and put back on the heat and bring to a simmer. Let simmer for 1 -1.5 hours stirring every now and again until golden tan in colour, thick and syrupy. Strain through a sieve into an airtight container, it will keep in the fridge for months.