Heat oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a bowl, whisk flour, baking soda, baking powder, salt, and cinnamon.
In another bowl, beat butter and sugar until light and fluffy. Add eggs one at a time, then vanilla.
Stir in mashed bananas and sour cream until combined.
Gradually add flour mixture to wet ingredients. Fold in walnuts.
Pour into pan, sprinkle with 2 tbsp sugar and extra walnuts. Bake 40–45 minutes, until a toothpick comes out clean.
Cool before slicing. Serve plain, dusted with powdered sugar, or with cream cheese glaze.
