1 cup (5 oz) all-purpose flour
3/4 cup (4 oz) whole-wheat flour
1/2 cup (4 oz) sugar
2 teaspoons baking powder
1 teaspoon baking soda
1 cup (8 fl oz) buttermilk
1 cup (8 oz) mashed banana
2 tablespoons canola oil
1 large egg
1 teaspoon vanilla extract
1/2 cup (2 oz) chopped walnuts
Preheat the oven to 375F. Spray 12 standard muffin cups with cooking spray or line with paper cups.
In a large bowl, stir together the flours, sugar, baking powder, and baking soda.
In another bowl, whisk together the buttermilk, banana, oil, egg, and vanilla. Pour the wet ingredients over the dry ingredients and stir until just blended. Spoon the batter into the prepared muffin cups, filling each about 3/4 full. Sprinkle the tops evenly with the walnuts.
Bake the muffins until they are lightly browned and a toothpick inserted in the center of a muffin comes out clean, 15 - 20 minutes. Let cool in the pan on a wire rack for 15 minutes, then turn out onto the rack and let cool completely.
I followed the recipe exactly except I probably used a teaspoon and a half of vanilla, and I omitted the nuts on the tops of the muffins since L doesn't really like them enough to make the calories and expense worth it to use them. And it made 20 muffins for me; my muffin pans must be smaller than standard because I couldn't have filled them more than I did without having overflow.
I really appreciated that the recipe gave measurements in both volume and weight. I very much prefer to weigh ingredients, especially flour, and love it when recipes provide it already and I don't have to convert cups to ounces.
Once I dig out the reference, I'll update the post to include where I found the recipe.