Line 2 large baking sheets with parchment paper; set aside.
Add the butter, sugar, milk, cocoa powder, and salt to a large saucepan and bring to a rolling boil. Boil for 1 minute, then remove from the heat.
½ cup unsalted butter, 2 cups granulated sugar, ½ cup milk, ¼ cup unsweetened cocoa powder, ⅛ teaspoon kosher salt
Stir in the peanut butter, vanilla, and cinnamon (if using); mix until incorporated.
½ cup creamy peanut butter, 2 teaspoons pure vanilla extract, ⅛ teaspoon ground cinnamon
Add the oats and mix will.
3 cups quick oats
Working quickly, drop by tablespoon onto the prepared baking sheets and let the cookies completely set/cool. If desired, flatten the mounds slightly with a spoon.
Store in an airtight container.