Combine sugar, milk and butter in a saucepan over medium-high heat.
Bring to a rolling boil then lower the heat to medium-low. Boil for 2 minutes stirring constantly.
Remove from heat and stir in peanut butter and vanilla. Stir until the peanut butter has completely melted.
Quickly stir in the oats, stirring until evenly coated. (If the mixture seems runny add a few more oats.)
Drop batter using a tablespoon or a small cookie scoop (1 1/2 Tbsp scoop) onto waxed paper. Use the back of a spoon to gently flatten the centers.
Let cool until set, then enjoy.
Notes
Creamy Peanut Butter: You could use chunky peanut butter in place of creamy peanut butter, using the same amount. I don't recommend using natural peanut butter for this recipe.
Peanut Butter Substitution: You could swap out the peanut butter with almond butter, sunflower seed butter, cookie butter or an alternate nut butter that suits your taste.
Sugar: You could swap out 1/2 cup of granulated sugar and use 1/2 cup of light brown sugar.
Chocolate Chips: You could add 1/2 cup mini chocolate chips to the oat mixture.
Glaze: You may opt to dip these cookies in warm chocolate or drizzle the tops with a simple ganache. When doing so, make sure the cookies are completely cool before handling them too much.
Troubleshooting: If the mixture becomes to stiff too quickly add milk in small increments, like 2-3 tablespoons at a time to loosen it up. Likewise, if it's too thin, add a few more oats and stir like crazy to combine.