Heat milk in a heavy-bottomed saucepot over medium heat until scalding (just before first boil),and the temperature on a digital thermometer shows 185°F (85°C).
Remove from heat and let cool until just warm to the touch and temperature reads 110°F (43°C), about 10–15 minutes.
Once cooled, whisk in yogurt, cover with a lid and place in a warm spot (the inside of an oven with the light left on, on top of a heating pad, or even wrapped in tea towels) for several hours or overnight. The residual heat from the pot helps with the incubation time. Note: fluctuations in kitchen temperatures can influence fermentation time.
Once thickened, stir and store in a clean glass jar in the fridge for up to 2 weeks, making sure to reserve 1⁄4 cup (60 mL) of this homemade yogurt for the next batch. Making yogurt at home, you will notice that it may not be as thick as store bought and also milder in flavour at first. The flavour will develop with each batch you make.