Preheat the oven to 350°F (175°C). Grease and flour two 8-inch round cake pans.
In a medium bowl, whisk together the flour, baking soda, salt, and ground cinnamon. Set aside.
In a large bowl, cream the 1/2 cup softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Mix in the mashed bananas and sour cream until combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until incorporated. If using, fold in the chopped walnuts.
Divide the batter evenly between the prepared cake pans and smooth the tops.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then turn out onto wire racks to cool completely.
To make the frosting, beat the 1 cup softened butter until creamy.
Gradually add the powdered sugar, 1 cup at a time, beating well after each addition.
Add the vanilla extract and milk, and continue to beat until the frosting is light and fluffy. Add more milk if needed to reach spreading consistency.
Once the cakes are completely cool, level the tops if necessary.
Place one cake layer on a serving plate and spread a layer of frosting evenly over the top.
Place the second cake layer on top and frost the top and sides of the cake with the remaining frosting.
Decorate the cake with sprinkles or banana slices as desired.
Refrigerate the cake for at least 30 minutes before serving to set the frosting.