Combine the butter, sugar, milk, and cocoa powder together in a medium (about 3-quart) saucepan over medium heat. Whisk constantly until the butter melts. Whisking occasionally, bring to a boil. Once boiling, stop whisking and allow to boil for 1 minute. If you want to be precise and you own an instant-read thermometer, the temperature should rise to 225°F (107°C).

Remove from heat, then immediately stir in the peanut butter and vanilla extract until completely combined. Finally, stir in the oats and salt.

Allow mixture to sit for 5 minutes. This allows the oats to soak up some moisture.

During the 5 minutes, line two baking sheets with parchment paper or silicone baking mats, and make sure there is enough room in your refrigerator for the baking sheets.

Using a 1-Tablespoon cookie scoop (or simply a Tablespoon), drop spoonfuls of dough onto the lined baking sheets. If desired, slightly flatten down with the back of a spoon. Repeat with the rest of the dough.

Refrigerate the cookies for 30–60 minutes so they set. Remove from the refrigerator and enjoy!

Cover and store leftover cookies in the refrigerator for up to 1 week.