Combine the chestnut flour, brown rice flour, corn starch, cocoa powder, brown sugar, baking soda, and salt in the bowl of a food processor (or in a large mixing bowl if making by hand).
Sprinkle the butter over the top and pulse 5 times. If mixing by hand, work the butter into the dry ingredients with your fingers or a pastry cutter, until the pieces are about the size of peas and almonds.
Add the honey, milk, and vanilla and pulse just until it comes together as a dough.
Place the dough in a mound on a large piece of parchment paper and flatten it into an even rectangle with your hands. Place another piece of parchment paper over the top and roll it out to a 10-inch by 6-inch rectangle until it is just under ¼-inch thick. Patch it up as needed to make sure the whole slab is even thickness. Use a pastry cutter to cut it into 15 2-inch squares, use the back of a skewer to poke five holes in the top of each square, and sprinkle coarse sugar over the top.
Slide the parchment paper onto a large baking sheet and freeze while you preheat the oven.
Preheat the oven to 350°F.
Remove the pan from the freezer and separate the graham crackers so there is at least 1-inch of space between them. Bake for 13-15 minutes, until the edges just begin to darken, being careful not to let them burn! Remove from oven and use a thin metal spatula to transfer the crackers to a wire rack to cool completely (they'll crisp up as they cool).
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.