Preheat the oven to 325 degrees F (165 degrees C). Grease and flour two (9-inch) round cake pans.
Make cake: Beat peanut butter and butter in a mixing bowl until light and fluffy. Add eggs, one at time, mixing well after each addition. Add cake mix alternately with water, stirring until just combined. Pour batter into the prepared pans.
Bake in the preheated oven until a toothpick inserted into the centers comes out clean, about 25 minutes. Allow cakes to cool in the pans for 10 minutes, then turn out onto a wire rack to cool completely before frosting.
Make frosting: Beat peanut butter and butter in a mixing bowl until light and fluffy. Add confectioner's sugar; mix until combined. Beat in enough cream until frosting reaches spreading consistency.