Heat oven to 375°F. Grease a 13 x 9 inch pan.

In a large bowl, beat together sugar, sour cream, butter, and eggs until creamy.
Blend in bananas and vanilla extract.
Add flour, baking soda, salt, and blend for 1 minute. Stir in walnuts.
Spread batter evenly into pan. Bake 25 minutes or until golden brown.

Cool slighty (warm but not hot) and frost with frosting.

For frosting,
Heat butter in a large saucepan over medium heat.
Watch carefully when it reaches a medium brown color turn off heat, whisk in powdered sugar at a time, adding milk as it thickens, then add in vanilla.
Pour over the warm banana bread brownies immediately, and smooth with a spatula, butter knife, or frosting spatula.
Cool completely before cutting.