Prep: Preheat oven to 350°F and prepare a cookie sheet by lining with parchment or coating with cooking spray. The cookies spread out quite a bit so it may be necessary to use more than one baking sheet or bake in batches.
Wet Ingredients: In a medium bowl, whisk together eggs, oil, vanilla, and peanut butter until fully combined.
Combine: Add cake mix to liquid ingredients and stir with a spatula or spoon until the dough just starts to come together. Add Reese’s cups/pieces and chocolate chips and continue to mix until no cake mix is visible.
Portion: Form round balls using 2 rounded tablespoons of dough and place 2-3 inches apart on the cookie sheet. Press cookie balls with the bottom of a glass or measuring cup until they are half their original height.
Bake: Bake for 13-15 minutes, rotating halfway through. Cookies will be done when they are set on the edges and golden on the edges and bottom.
Store: Let cool for a couple of minutes on the cookie sheet before transferring to a wire rack to let cool completely. Store in an airtight container at room temperature for 3-4 days or in the freezer for up to 3 months.
Freeze for Later: The cookies can be frozen before or after they are baked and kept in the freezer for up to 3 months. Store in an airtight container or ziploc bag. If they have been baked, make sure they are fully cooled first.
Prepare from Frozen: Bake frozen cookie dough according to the recipe instructions - baking times will still be the same. For baked cookies, let thaw to room temperature and enjoy.