Found this on line but changed it to suit what ingredients I had on hand. I made it the other day and took it to work. Everyone liked it so I guess it's worth posting here.
1 package yellow cake mix
1 1/2 cups water
1/3 cup smooth peanut butter
1/3 cup vegetable oil
1 teaspoon vanilla
1. Preheat oven to 350 degrees. Grease and flour 2 9 inch round cake pans.
2. Put all cake ingredients in large mixing bowl. Mix as directed on box.
3. Pour into pans and bake for 23 to 28 minutes or until toothpick comes out clean.
4. Remove from oven and let cool in pans 10 minutes. Remove from pans and allow to cool on wire racks completely.
5. Prepare frosting: Beat together 1 can chocolate frosting and about 1/2 cup creamy peanut butter. Spread frosting on cake and garnish with chopped peanut butter cups if desired.