In a medium saucepan, combine sugar, butter, milk, and cocoa powder. Heat over medium heat, stirring occasionally, until butter is melted and mixture is smooth.
Bring mixture to a boil and continue to stir frequently. Once boiling, let it cook for 1 minute.
Remove saucepan from heat and immediately stir in peanut butter and vanilla extract.
Add oats and salt to the peanut butter mixture; stir until fully combined.
Drop tablespoonfuls of the mixture onto wax paper or a silicone baking mat. Let cookies cool completely to set.
Notes
For a crunchier texture, you can use chunky peanut butter. Feel free to add your favorite extras like coconut flakes or chopped nuts!