For the soaker, combine 1 cup (120g) flour, 1 ¼ cups (75g) bran flakes and 1 ¼ cups (315g) buttermilk. Stir to combine then cover with plastic or a tight fitting lid and refrigerate overnight (8–10 hours).
When ready to bake, heat oven to 425ºF (218ºC). Coat a muffin pan with nonstick cooking spray, then line 10 muffin wells with paper liners and coat liners with nonstick spray.
In a small bowl, combine 2 tablespoons (20g) cornstarch, 2½ teaspoons (11g) baking powder, ¾ teaspoon cinnamon, ¼ teaspoon baking soda, and ¼ teaspoon salt.
In a large bowl, whisk together ½ cup (160g) maple syrup, ⅓ cup (76g) oil, 1 egg and 1 teaspoon vanilla until smooth.
Add soaker to maple syrup mixture and whisk until thoroughly combined (start by using your whisk to break up the firm bran mixture—keep working it. It will loosen and come together.).
Whisk in starch mixture until combined then stir in ½ cup (56g) walnuts.
Divide batter evenly between 10 prepared cups.
Bake muffins for 8 minutes (at 425ºF) then turn oven down to 375ºF (190ºC) and continue baking until tops are set, center looks done and a toothpick inserted in center comes out with few crumbs attached, 15–17 minutes more (start checking them at 13 minutes to be safe).
Total bake time will be between 21 and 25 minutes.
Let muffins cool in muffin tin on wire rack for 10 minutes. Remove muffins from muffin tin and let cool 5 minutes longer. Serve.