Line a large colander or sieve with 5 layers of dampened cheesecloth, allowing the cheesecloth to extend over outside edges of colander; place colander in a large bowl.
Combine milk and buttermilk in a large, heavy stockpot. Attach a candy thermometer to edge of pan so that thermometer extends at least 2 inches into milk mixture. Cook over medium-high heat until candy thermometer registers 170° (about 20 minutes), gently stirring occasionally. As soon as milk mixture reaches 170°, stop stirring (whey and curds will begin separating at this point). Continue to cook, without stirring, until the thermometer registers 190°. (Be sure not to stir, or curds that have formed will break apart.) Immediately remove pan from heat. (Bottom of pan may be slightly scorched.)
Using a slotted spoon, gently spoon curds into cheesecloth-lined colander; discard whey, or reserve it for another use. Drain over bowl for 5 minutes. Gather edges of cheesecloth together; tie securely. Hang cheesecloth bundle from kitchen faucet; drain 15 minutes or until whey stops dripping. Scrape ricotta into a bowl. Sprinkle with salt; toss gently with a fork to combine. Cool to room temperature.
Note: Store in refrigerator up to 4 days.