Add the butter, sugar, milk, and salt to a medium saucepan over medium heat, stirring occasionally to help dissolve the sugar.
Once boiling, let the mixture boil for 2 minutes. It's best to set a timer when the mixture reaches a full boil.
Stir in the vanilla and peanut butter until well-combined, and then remove from the heat.
Add the oats and peanut butter mixture to a large bowl, and stir to combine.
This is where you need to keep a close watch – the mixture needs to be cool enough to handle, but still hot enough to stick together into cookies. As soon as you can touch the mixture without it being too hot, start to roll it into cookies and work fairly quickly. Use a slightly heaping 1.5-tablespoon scoop to measure out the dough, roll the dough into balls between your palms, and then slightly flatten the balls into circles about 2 inches in diameter. Place the shaped cookies onto a parchment lined baking tray and continue until they’re all shaped.
Let the cookies set because they will harden as they do. This takes about 20 minutes at room temperature.
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Notes
Have all your mix-in ingredients ready to go (oats, peanut butter, vanilla, and salt) so the mixture doesn’t begin to set up prematurely before you’ve have the chance to scoop them into balls.
If you only have salted butter on hand, omit the 1/4 teaspoon of salt called for in the recipe ingredients.
You can substitute with chunky peanut butter if desired, however, I would not recommend natural peanut butter for this recipe as they tend to be too runny and may cause the cookies to not set up.
It's best to set a timer for exactly 2 minutes ONCE it reaches a boil, NOT when it starts to bubble, which is considered a gentle boil (like a simmer). If you start your timer too soon, the cookies won't set properly.
Store your cookies in an air-tight container kept at room temperature for 3-4 days. Store in a cool, dry place away from direct sunlight.
To Freeze - allow the cookies to fully set then transfer them to an air-tight container or a large ziptop bag. Store in the freezer for up to 3 months.