Spread bran flakes on a baking sheet in an even layer and toast in oven for 6 minutes, stirring halfway through; let cool.

Bring dates and orange juice to a simmer in a saucepan over medium-high heat, reduce to medium-low and cook until juice has been absorbed, about 10 minutes.

Meanwhile, process bran cereal in a food processor until finely ground into a flour. Add toasted bran flakes, flour, baking powder, baking soda, and salt. Process bran mixture to combine; transfer to a bowl.

Add cooked dates and coconut oil to now empty food processor and puree. Add buttermilk, eggs, molasses, vanilla, and zest and pulse until combined, about 10 pulses.

Pour date mixture over bran-flour mixture and stir just until combined; fold in walnuts. Divide batter between paper-lined wells.

Bake muffins until a toothpick inserted into the centers comes out clean, about 25 minutes. Cool muffins in pan 2 minutes, then transfer to a wire rack to cool completely.