Cook: Add milk to a microwave safe bowl, then microwave on high for 3 to 5 minutes (if you have a kitchen thermometer, milk should reach between 185°F and 200°F).
Curdle: Add vinegar or lemon juice, stirring briefly to combine. Let sit undisturbed for 1 to 2 minutes. Milk should separate into white curds and yellowish liquid (test it by dragging a spoon through the bowl). If milk doesn’t curdle and the liquid is still white, add another tablespoon of lemon or vinegar and microwave for another 30 seconds.
Strain: Lay a clean cloth or sturdy paper towel in a wire mesh sieve, and set over a large bowl. Pour the milk mixture into the sieve to strain out the liquid whey. You can let your ricotta strain up to 1 hour. The longer it sits, the firmer your ricotta will be (though I like it fresh and warm, strained for just a few minutes). Stir salt into ricotta when finished straining.
Store ricotta in an airtight container in the fridge for 4 to 5 days.
Use the leftover liquid whey as a protein boost in smoothies! (This is why we don't add the salt until last, so your whey doesn't get salty and can be added to sweet or savory dishes.)