Prepare the Oven and Pan: Position a rack in the center of the oven and preheat to 375°F (190°C). Line 16 muffin cups with liners.
Toast Pecans: Place the chopped pecans in a small, dry skillet over medium heat. Toast for 3–5 minutes, stirring often, until fragrant. Remove from heat immediately and set aside.
Hydrate the Bran: In the medium bowl, pour the buttermilk over the wheat bran. Stir to combine, then set it aside to soften and "bloom" for about 10 minutes.
Combine Dry Goods: In the large mixing bowl, thoroughly whisk together both whole-wheat flours, baking powder, baking soda, ground cardamom, and cinnamon.
Mix the Wet Ingredients: In a separate bowl, use an electric mixer or a whisk to beat the oil, dark brown sugar, and maple syrup until well-combined. Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
Unite the Batters: Pour the hydrated bran mixture and the mashed bananas into the egg and sugar mixture, stirring gently with a spatula to combine.
Fold and Finalize: Create a depression in the center of the dry ingredients. Pour the combined wet ingredients into the depression. Gently fold the batter with a rubber spatula until just a few streaks of flour remain—do not overmix. Fold in the dark chocolate chips.
Portion and Bake: Divide the batter evenly among the 16 prepared muffin cups; they will be very full. Sprinkle the toasted pecans evenly over the tops. Bake for 22–28 minutes, or until the tops are domed and a wooden skewer inserted into the center of a muffin comes out with moist crumbs attached, but no wet batter.
Cool and Enjoy: Let the muffins cool in the tin for 10 minutes before carefully transferring them to a wire rack to finish cooling. Serve warm or at room temperature.