Preheat oven to 350*. Line 24 muffin tin cups with paper liners (or about 55+ mini muffins).
Beat bananas with a mixer at medium speed until mashed. Mix in brown sugar, walnut oil and egg until smooth. Mix together flour, baking soda, baking powder and salt and add to banana mixture. Beat yogurt and vanilla into mixture until smooth. Fold in walnuts. Fill muffin liners ¾ full; bake until tester comes out clean, about 20 minutes (or 14 for minis).