Warm the milk. Pour the milk into a clean, heavy-bottomed pot. Slowly warm to 86°F, stirring gently for even heating. Do not heat the milk to 185°F for this cultured, rennet-set method.
Add the culture. Stir in the cultured buttermilk gently and thoroughly. Hold the milk near 86°F for about 30 minutes to allow the culture to begin working.
Dilute and add the rennet. Immediately before using, mix 1/8 teaspoon double-strength microbial rennet with 1/4 cup cool, non-chlorinated water. Add to the milk and stir gently with an up-and-down motion for about 30 seconds. Stop stirring.
Culture and set. Cover the pot and leave it undisturbed at a comfortable room temperature, ideally around 72–78°F, until a firm curd forms. Begin checking around 5–6 hours; approximately 6–8 hours is a useful guideline. Judge readiness by the curd, not the clock.
Check for a clean break. Insert a clean knife or finger into the curd at an angle and lift gently. The curd should split cleanly and whey should appear in the opening. If the curd is still very soft, allow more time.
Cut the curd. Cut into approximately 1/2-inch cubes. Make vertical cuts in one direction, rotate the pot, cut again, then make horizontal cuts as evenly as practical. Let the cut curds rest for 10 minutes.
Cook slowly. Gradually warm the curds and whey to 115–120°F over approximately 30–45 minutes. Stir very gently, especially at the beginning. Slow heating helps the curds release whey without becoming tough or rubbery.
Check the texture. Taste a curd. It should hold together and have a tender bite. Continue cooking gently if the curds are too fragile. Stop when the texture is where you want it.
Drain and rinse. Transfer the curds carefully to a colander and drain the whey. Gently rinse with progressively cooler water, finishing with cold water. This stops the cooking, cools the curds, and removes excess whey and acidity. Drain thoroughly.
Salt and cream. Transfer the cooled, drained curds to a clean bowl. Add 1 teaspoon kosher salt, taste, and adjust as needed. For creamy cottage cheese, gently fold in 1/2 cup cold heavy cream. Add additional cream, up to about 3/4 cup total, if desired.
Chill. Transfer to a clean covered container and refrigerate promptly. The texture will firm slightly as the cottage cheese chills.