Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans or one 9×13 inch pan.

In a large bowl, combine the chocolate cake mix, eggs, water, oil, peanut butter, and vanilla. Beat with an electric mixer for about 2 minutes until well combined.

Divide the batter between your prepared pans if making a layer cake, or pour it all into the 9×13 pan.

Bake until done. For 8-inch rounds, about 25-30 minutes; for a 9×13 pan, about 30-35 minutes. A toothpick inserted in the center should come out with a few moist crumbs, but no wet batter.

Cool completely before frosting. For layer cakes, cool in pans for 10 minutes, then turn out onto wire racks to finish cooling.

For the frosting, beat the butter and peanut butter together until smooth and creamy, about 2 minutes.

Add powdered sugar, salt, and vanilla. Mix on low speed until incorporated.

Add milk one tablespoon at a time until you reach your desired consistency. For spreading on a cake, you want it fluffy but spreadable.

Frost your cooled cake. For layer cakes, place one layer on your serving plate, spread with frosting, top with the second layer, and frost the top and sides.