Add butter to a medium-sized saucepan over medium heat.
Once butter has melted, add sugars, milk, and cocoa powder. Bring to a boil and let boil for 1 minute, stirring occasionally.
4 Tbsp. butter, ¾ cup granulated sugar, ½ cup milk, ¼ cup cocoa powder, ½ cup brown sugar
Remove from the heat and stir in peanut butter, vanilla, and salt.
⅔ cup peanut butter, 2 tsp. vanilla extract, ¼ tsp. salt
Pour sauce over the oats in a large bowl and mix to combine. Let sit for 5 minutes before scooping out into 1 ½ tablespoon-sized balls.
3 cups old-fashioned oats
Place balls evenly spaced on a parchment paper-lined baking sheet, pressing down slightly to flatten them out. Refrigerate for at least 1 hour or up to overnight for the best results.