Measure bran, flours, corn starch, brown sugar, baking powder, flax meal, xanthan gum, salt, baking soda, and spices into the bowl of your food processor, blitz to combine.
Add molasses, honey, and vanilla extract, blitz once more to combine.
Add butter, blitz a few times until mixture resembles gravel.
Stream in milk as you run the food processor, just long enough to start to bring it together as a dough – you may need to use a little more or less milk. Do NOT over-process it!
Remove dough from processor, knead lightly to bring it together as a ball. Wrap in plastic film, chill for 1 hour.
Preheat oven to 350 F (180 C), line baking sheets with parchment paper
Generously sprinkle clean work surface with corn starch, roll dough to 1/8" thick.
Use cookie cutters or a pizza wheel to cut out squares or rectangles, place graham crackers 1" apart on greased baking sheets.
Prick top of each cookie several times with a fork.
Mix together sugar and cinnamon, sprinkle over prepared cookies.
Bake for about 12 minutes.
Allow cookies to cool for 5 minutes before removing from baking sheets. Cool completely before storing in an airtight container