Mix and Bake the Chocolate Cake: Prepare 1 box chocolate cake mix according to the package for a 9×13-inch pan, using the required eggs, oil, and water. Then pour into a greased pan and bake as directed (typically 28–35 minutes). The cake is done when a toothpick inserted in the center comes out mostly clean with a few moist crumbs.

Cool Completely: Set the pan on a wire rack. Next, allow the chocolate cake to cool to room temperature. This step prevents the frosting from softening or sliding.

Start the Peanut Butter Frosting: In a mixing bowl, combine ½ cup margarine (softened) and ¾ cup Reese’s creamy peanut butter. Beat until perfectly smooth and creamy, scraping the bowl as needed.

Finish the Frosting: Add 2 cups powdered sugar, 2 Tablespoons milk, and 1 teaspoon vanilla extract. Then beat until the frosting turns fluffy and spreadable. If it’s too thick, add milk ½ Tablespoon at a time. If it’s too thin, add a tablespoon of powdered sugar and beat again.

Frost with a Light Hand: Dollop frosting across the cooled cake. Afterward, gently spread with an offset spatula. Avoid pressing too hard or dragging, which can lift crumbs and mar the finish.

Slice and Serve: Cut into neat squares. Serve the Chocolate Cake with Peanut Butter Frosting at room temperature for the softest crumb and the best flavor.