Pour whole milk into a heavy-bottomed saucepan and heat gently over medium-low heat, stirring occasionally to prevent scorching.

Monitor temperature with digital thermometer until milk reaches 185°F (85°C). Do not let it boil vigorously as this can create tough curds.

Remove from heat and slowly stir in fresh lemon juice or vinegar. You'll immediately see curds forming and separating from the whey.

Let the mixture sit undisturbed for 10-15 minutes to allow complete curd formation and separation.

Line a fine-mesh strainer with cheesecloth or clean kitchen towel, place over a large bowl, and carefully pour the mixture through.

Gather the corners of the cheesecloth and tie into a bundle. Let drain for 2-4 hours for soft quark, or 6-8 hours for firmer consistency.

Transfer drained quark to a clean container, stir in optional salt if desired, and refrigerate until ready to use.