In a large bowl, cream together the sugar and butter.
Add the eggs, milk and vanilla. Set aside.
In another bowl, combine the flour, baking soda, baking powder, salt and cream of tartar. Mix well.
Add the flour mixture to the wet ingredients and mix well, then fold in the bananas and walnuts.
Pour the batter into the prepared cake pan and smooth it to cover the bottom of the pan evenly.
Bake for 40 minutes, or until the middle of the cake springs back when you touch it.
Allow the cake to cool while you prepare the frosting.
Using an electric mixer, mix the butter and cream cheese, scraping down the sides as needed, until the mixture is light and fluffy. This will take about 3 minutes.
With the mixer on low speed, add the powdered sugar, 1/2 a cup at a time, until it is thoroughly mixed.
Add the vanilla and beat the frosting on medium speed for about 2 minutes.
Spread the frosting on the cooled cake, then top with chopped walnuts.
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.