Bring sugar, butter, milk, cocoa, and salt to a full rolling boil in a saucepan for 2 minutes.
Add quick-cooking oats, peanut butter, and vanilla; mix well.
Working quickly, drop by teaspoonfuls onto waxed paper and let cool.
Note: I have been making a version of these cookies for over 50 years since my aunt Ida taught me how to make them when I was 8 years old. There are a couple of things that I suggest to make them easier and better. First, don't use margarine. These are far better with butter. Second, use 5 tablespoons of cocoa rather than 3. For ease of mixing, add the peanut butter when you take the boiling mixture off the heat. Completely mix it in before you add the vanilla. Adding vanilla immediately causes the alcohol in the vanilla to boil and evaporate and the flavor isn't as good. Finally, add the oats. I use parchment paper rather than wax paper and I don't use cookie sheets - I just lay the paper on the countertop.