Preheat oven to 350°F.
For the cake, put cake mix, mashed bananas, egg whites, and water in a large bowl. Beat well with a mixer for 2 minutes. grease two 9 inch cake pans, and pour batter into pans. Pour one pan more than the other, about 1 inch. Bake for 30 minutes.
For Filling place peanut butter, powered sugar, and butter in a small mixing bowl. Cream until the ingredients are incorporated, and set aside.
For frosting combine 1/2 cup butter and cococa in a medium bowl, and mix together. Alternate the powered sugar and milk by adding the powered sugar fist. Then stir in vanilla.
Once the cake is cool, place the thinner one on the bottom. Spread half of the peanut butter filling on top. Cut the bigger cake in half. Place one on top of the peanut butter topped cake. Trim edges of cakes with a knife if necessary. Spread the remaining peanut butter filling on top of the second layer. Place the third cake layer on the top, and trim again. Spread frosting evenly over the top and the edges of the cake. Place slivered almonds around the bottom of the cake and in the center.