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. Place the chopped pecans in a small, dry skillet over medium heat and 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 while you prepare the other components. This step ensures a moist, tender crumb.
Combine Dry Goods : In the large mixing bowl, thoroughly whisk together the whole-wheat flour, all-purpose flour, baking powder, baking soda, ground cardamom, sea salt, and cinnamon.
Mix the Wet Ingredients : In a separate bowl, use an electric mixer or a whisk to beat the melted butter, 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 chunks.
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.