Preheat oven to 350 degrees.
Spread the 1/2 cup brown sugar evenly in a 9 x 9 inch pan.
Cut the 3 tbsp of butter into 9 pieces and place them through out the pan.
Put in the oven for 6-8 minutes. In the mean time prepare the batter.
Combine the flour, sugar, baking soda, salt, cinnamon and nutmeg in a mixing bowl.
Add all the wet ingredients: eggs, mashed bananas, sour cream, and oil.
Mix together the dry and wet ingredients but do not over mix. I did about half a minute on the first speed of the kitchen aid to incorporate all the ingredients and then a minute or 2 on the second-fourth setting.
Fold in the walnuts.
When the buttered sugar crust is ready, take it out of the oven and make sure it is spread evenly. It can cool and harden if you haven't finished the batter but it should be evenly spread. You can also toss a few walnuts on top of it so they get incorporated into the crust.
Pour the batter on top of the sugar crust and spread evenly.
Bake for about 22 minutes or until a toothpick comes out clean.
Run a knife along the edge of the pan, it should separate with no issues. Flip the cake onto a plate or different pan.
Any sugar that is still stuck to the pan can be scraped off with a spoon and crumbled on top (it's the best part)!
Let cool, sprinkle with powdered sugar (if you want) and enjoy.