Fill the sink with cold water and place a couple of ice packs in the water.
Place the milk and milk powder in a saucepan and stir to combine over a medium heat. Place the thermometer into the saucepan and stir occasionally while you bring the milk up to 90 degrees celsius (194 Fahrenheit) and hold at this temperature for 5 minutes.
Remove the saucepan from the heat and place in the sink and stir as you bring the temperature down to 46 degrees celsius (115 Fahrenheit). Remove the saucepan from the water and use a whisk in the yoghurt until combined. Pour into a thermos and seal and leave for 6 hours. Decant the yoghurt into jars or sealed containers and refrigerate.
Notes
If you notice your yoghurt is a little lumpy as you decant it from the thermos simply whisk for a creamier texture. Yoghurt can also be made with skim milk, although the consistency is slightly thinner.For a thicker yoghurt strain through muslin for 2 hours.For a stronger tasting yoghurt leave for 12 hours before decanting.