Step 1: In a medium saucepan, combine the unsalted butter, granulated sugar, milk, and unsweetened cocoa powder over medium heat. Stir continuously until the butter is fully melted, and the sugar has dissolved, creating a smooth, uniform chocolate mixture.
Step 2: Increase the heat slightly to bring the mixture to a full, rolling boil that cannot be stirred down. Once a vigorous boil is achieved across the entire surface, start timing for exactly one minute. Continue to stir gently to prevent scorching.
Step 3: Immediately remove the saucepan from the heat. Stir in the creamy peanut butter, vanilla extract, and a pinch of salt. Mix briskly until the peanut butter is completely melted and incorporated, and the mixture is glossy and smooth.
Step 4: Add the 3 cups of quick-cooking oats to the hot mixture. Using a sturdy spoon or spatula, stir vigorously until all the oats are thoroughly coated and no dry spots remain. The mixture should be thick and spoonable.
Step 5: Line a baking sheet or countertop with parchment paper or a silicone mat. Working quickly, drop rounded tablespoonfuls of the oat mixture onto the prepared surface, spacing them about 2 inches apart.
Step 6: (Optional) For a flatter, more uniform cookie, gently press the top of each mound with the back of a spoon to create a ½-inch-thick disk.
Step 7: Allow the cookies to cool and set at room temperature for 20 to 30 minutes. They are ready when the surface is no longer shiny and they can be lifted cleanly from the paper without sticking.
Step 8: Once fully set, transfer the cookies to an airtight container. Layer with parchment or wax paper to prevent sticking. Store at room temperature for up to one week.