Preheat oven to 350 degrees F. Grease/Flour a 9x13 inch pan. Mix 2/3 cup buttermilk, 3 eggs, 1/2 cup butter, and vanilla extract. Set aside.
In a large bowl combine cake mix, baking soda. Gradually add the wet ingredients in step 1 to the dry mixture. Beat at low speed until moist then beat at medium speed for 3 minutes. Slice the bananas. Fold the bananas and the sour cream into the wet mixture. Pour into the prepared pan. Bake in preheated oven for 40 to 45 minutes, or until cake springs back when lightly tapped. A toothpick inserted may leave a slight crumb. Allow to cool on wire rack.
Frosting: mix cream cheese, butter, vanilla, salt and gradually add powdered sugar. Mix well, and fold in the pecans. Frost cake and enjoy!