Preheat oven to 400 degrees F for muffins, mini loaves and mini Bundts or 375 degrees F for a large loaf. (See pan options, below.) Coat pan(s) with cooking spray.

Whisk whole-wheat flour, all-purpose flour, baking powder, cinnamon, baking soda, nutmeg and salt in a large bowl. Whisk eggs, buttermilk, brown sugar, butter, oil and vanilla in another large bowl until well combined.

Make a well in the dry ingredients and stir in the wet ingredients until just combined. Add bananas, walnuts and chocolate chips. Stir just to combine; do not overmix. Transfer the batter to the prepared pan(s). Top with additional walnuts, if desired.

Bake until golden brown and a skewer inserted in the center comes out clean, 22 to 25 minutes for muffins or mini Bundts, 35 minutes for mini loaves, 1 hour 10 minutes for a large loaf. Let cool in the pan(s) for 10 minutes, then turn out onto a wire rack. Let muffins and mini Bundts cool for 5 minutes more, mini loaves for 30 minutes, large loaves for 40 minutes. Pan options: 1 large loaf (9-by-5-inch pan) 3 mini loaves (6-by-3-inch pan, 2-cup capacity) 6 mini Bundt cakes (6-cup mini Bundt pan, scant 1-cup capacity per cake) 12 muffins (standard 12-cup, 2 1/2-inch muffin pan)

Ingredient note: Whole-wheat pastry flour, lower in protein than regular whole-wheat flour, has less gluten-forming potential, making it a better choice for tender baked goods. You can find it in the natural-foods section of large super markets and natural-foods stores. Store in the freezer.

Tips: No buttermilk? You can use buttermilk powder prepared according to package directions. Or make "sour milk": mix 1 tablespoon lemon juice or vinegar to 1 cup milk.

To toast chopped walnuts, place in a small dry skillet and cook over medium-low heat, stirring constantly, until fragrant and lightly browned, 2 to 4 minutes.

MAKE AHEAD TIP: Store, individually wrapped, at room temperature for up to 2 days or in the freezer for up to 1 month.