In large bowl, whisk together flour, baking soda, salt and cinnamon.
In separate bowl, beat together softened butter, sugar, sour cream, vanilla and eggs until smooth.
Beat in the mashed bananas until well combined.
Gradually mix in the dry ingredients until well combined
Fold in the 1/2 cup chopped walnuts.
Pour batter into prepared pan.
Bake 25-30 minutes or until golden brown and toothpick comes out clean.
Once cooked through, allow to cool 15 minutes.
To make frosting, melt butter over medium heat for 2-4 minutes. Watch the butter carefully, once it starts to turn a slight brown color and smell like roasted nuts, then its ready. Be careful not to burn the butter.
Remove butter from heat and whisk in the powdered sugar, milk and vanilla. Add in 1-2 more tbsp milk to thin out the frosting.
Spread frosting over brownies and top with chopped walnuts.
Allow frosting to harden for an hour before cutting and enjoy!