Preheat oven to 400 °F for muffins, mini loaves and mini Bundts or 375 °F for a large loaf. Coat pan(s) (see "Pan Options") with cooking spray.
Prepare Quick-Bread Dry Mix.
Whisk eggs, buttermilk, brown sugar, butter, oil and vanilla in a large bowl until well combined.
Make a well in the center of the dry ingredients, pour in the wet ingredients and stir until just combined. Add mango and pecans. Stir just to combine; do not overmix. Transfer batter to the prepared pan(s). Top with additional pecans, if desired.
Bake until golden brown and a wooden skewer inserted into 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.
Make Ahead Tip: Store, individually wrapped, at room temperature for up to 2 days or in the freezer for up to 1 month.
No buttermilk? Mix 1 tablespoon lemon juice into 1 cup milk.
To cut a mango: Slice both ends off the mango, revealing the long, slender seed inside. Set the fruit upright on a work surface and remove the skin with a sharp knife. With the seed perpendicular to you, slice the fruit from both sides of the seed, yielding two larger pieces. Turn the seed parallel to you and slice the two smaller pieces of fruit from each side. Cut the fruit into the desired shape.
To toast walnuts, pecans or hazelnuts, spread nuts onto a baking sheet and bake at 350°F, stirring once, until fragrant, 7 to 9 minutes. To toast sliced almonds, cook in a small dry skillet over medium-low heat, stirring constantly, until fragrant and lightly browned, 2 to 4 minutes.
Each of these recipes will make:
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)
Storage smarts: For long-term freezer storage, wrap your food in a layer of plastic wrap followed by a layer of foil. The plastic will help prevent freezer burn while the foil will help keep off-odors from seeping into the food.