In a large saucepan, combine butter, sugar, salt, evaporated milk, and cocoa.
Stir over medium-high heat until smooth (about 2 minutes).
Bring to a FULL boil and boil for 3 minutes.
Stir once or twice; must boil for exactly 3 minutes.
Remove from heat, add peanut butter and vanilla, and stir until smooth.
Mix in oats until well combined.
Let the mixture sit for 3 minutes to semi-firm.
Use a tablespoon to drop the mixture on parchment-lined baking sheets.
Mixture firm up in about 1 hour or refrigerate.
Alternatively, spread in an 8 x 8-inch pan, chill, then cut into squares.