Prepare 2 8inch or 2 9inch round cake pan,greased and floured. Preheat oven to 350. In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time beating well after each. Beat in the bananas and vanilla. Combine the flour, baking powder, baking soda and salt; add to the creamed mixture alternately with the buttermilk, beating well after each addition. Fold in 3/4 cup of pistachios. Transfer to prepared pans. Bake for 45 to 55 minutes. I check after 40 due to ovens varying. Test. Cool in pan at least 15 minutes before removing. Cool completely before frosting.
Frosting: In a large bowl beat cream cheese and butter until smooth. Add cocoa; mix until blended. Add confectioners sugar and vanilla beat until creamy. After both layers and middle are frosted if you want I put chopped pistachios just around the bottom edge of the cake.(optional) I also use a pastry bag and a petal pastry tip to make it a little fancier on top of the cake.(optional)