Preheat oven to 350°F and prepare a cookie sheet with parchment paper or a non-stick silicone baking mat.

Pour cake mix into a large mixing bowl and whisk to remove any lumps.

Add 1 cup peanut butter, eggs, and oil to the bowl with cake mix. Beat together using a stand mixer or electric hand mixer until all ingredients are well incorporated (the cookie batter will be thick).

Gently press down cookies to flatten.

Bake at 350°F for 9-11 minutes. Keep an eye on them and take care not to over-bake.

Remove cookies from oven and allow to cool on the cookie sheet for about 10 minutes before moving to a wire cooling rack.

Add the butter, peanut butter, and milk to a small saucepan over medium heat. Bring to a boil, whisking together ingredients, then remove from heat.

Slowly add powdered sugar, continuing to whisk until the frosting is smooth. Stir in the vanilla.

Pour frosting over cookies, slowly as to make sure that it coats the surface of each cookie.

Allow frosting to set, then enjoy!