Line two baking sheets with parchment paper, silicone liners, or waxed paper.
Place the butter, sugar, milk, and cocoa powder in a large saucepan. Cook over medium heat, stirring often. Bring to a boil, and continue cooking until the mixture reaches 230°F (110°C). (About 60 to 90 seconds.)
Remove the pan from the heat, and stir in the vanilla and salt. Then stir in the oats and peanut butter until thoroughly combined.
Drop onto the prepared pans by 2 tablespoon portions. (A #30 scoop works well.) Allow to cool completely before serving.
Notes
Store in an airtight container at room temperature up to 3 or 4 days.