Preheat oven to 350°F (175°C) and grease two 9-inch round cake pans.
In a large mixing bowl, sift together flour, sugar, baking soda, and salt.
In a separate medium bowl, whisk together mashed bananas, eggs, oil, buttermilk, and 1 teaspoon of vanilla extract.
Gently fold the wet ingredients into the dry ingredients until just combined; do not overmix.
Fold 3/4 cup of the toasted walnuts into the batter.
Divide the batter evenly between the prepared cake pans.
Bake for 30 to 35 minutes or until a skewer inserted into the center comes out clean.
Allow cakes to cool in pans for 10 minutes before transferring to a wire rack to cool completely.
In a clean bowl, beat cream cheese and butter until smooth and creamy.
Gradually add powdered sugar and the remaining 1 teaspoon of vanilla extract, beating until the frosting is light and fluffy.
Place one cake layer on a serving plate and spread a layer of frosting over the top.
Place the second cake layer on top and cover the top and sides of the cake with the remaining frosting.
Garnish the top and sides of the cake with the remaining 1/4 cup of toasted walnuts.
Refrigerate for at least 30 minutes before slicing to allow the cream to set.