Cut out three pieces of cheesecloth into 12-inch squares.
Line a colander or medium strainer with all three layers of cheesecloth.
Set colander in sink.
Combine milk, buttermilk, salt, lemon zest and thyme in a large heavy-bottomed saucepan, and heat over medium-high heat, stirring often until mixture has separated into white curds and translucent whey, about 8 minutes.
If using lowfat buttermilk, separation occurs at about 180 degrees F and the curds will clump together readily.
If using whole buttermilk, separation occurs closer to the boiling point, about 212 degrees F, and the curds are finer-grained.
When using whole buttermilk, let curds and whey stand off heat for about 3 minutes after separation, so the curds cling together-this way you won't loose tiny curds through the cheesecloth.
Ladle the contents of the saucepan into the prepared colander.
Let the whey drain, 1 to 2 minutes.
Lift the four corners of the cheesecloth and gather them together.
Gently twist the gathered cloth over the cheese and press out any excess whey. Do not squeeze to tightly or you will end up with a grainy cheese.
Cheese can be unwrapped immediately and served warm, or let stand until cooled to room temperature, about 10 minutes more.
To serve a firmer cheese, transfer cheese, in its cloth, to a small flat-bottomed dish or pie plate; refrigerate until cool, about 10 minutes.
Unwrap cheese and gently invert onto plate; discard cloth.
Tent cheese with plastic wrap and keep refrigerated up to 2 days.
Remove from refrigerator and let stand for 10 minutes at room temperature before serving.
You can omit the lemon and thyme and opt for cracked black pepper, or fresh rosemary, or red pepper flakes, lemon zest and dill -- whatever fresh herbs you like.
You can also reduce salt to 1/2 teaspoon, adding 2 teaspoons pure vanilla extract and 1 to 2 tablespoons sugar, depending on how sweet you want your cheese.