Leave milk and buttermilk on the counter until they reach room temperature (I leave mine overnight).
Combine milk and buttermilk in a deep pan and place over low heat. Heat the mixture up until it’s warm when you touch it with your finger. Stir only slightly.
Remove from heat, cover with a lid and leave in warm place for 24 hours (I leave it on the counter next to the stove)
In 24 hours place the pan over low heat again. Heat up until it’s warm to touch. Stir a little to make sure the mixture is warm throughout. Remove from stove and leave for another 24 hours in warm place.
In 24 hours heat up the mixture for the last time. At this point cheese should completely separate from whey (greenish liquid). (*In rare case when the whey doesn't separate after 48 hours, heat up the mixture over low heat until it does. At higher temperatures separation happens much faster.)
Use a fine strainer lined with cheese cloth to strain cottage cheese. Leave it in strainer for 5-6 hours or overnight to make sure that all whey has drained.
After it's done, keep cottage cheese refrigerated.