Ladle out any butter lumps. Slowly pour rennet mixture in a spiraling pattern over milk, stirring. As you check the temperature, insert the thermometer gently to avoid breaking clot more than necessary. Next, cut through the solid clot to the pan bottom with a long knife, to release clear- colored whey. From this point on, you need clean but not sterilized equipment.
Ladle curds into colander.
Let stand until curds stop dripping, about 1 hour. Each day, replace cheesecloth and discard whey. If after 15-30 seconds the slice begins to soften and melt and, when held by one end, the piece stretches from its own weight, it's ready. If the slice doesn't stretch but tears, chill the remaining curd, testing daily, up to 4 days. If curd still won't melt, the milk got too hot or sufficient acidity did not develop.