Grind sugar in a spice grinder until fine, about 20 seconds; transfer to the bowl of a food processor. Sift together graham flour, pastry flour, baking powder, baking soda, and salt (it may look like some of it won’t sift through, be patient and try to get as much through as you can. Then turn whatever is left out out into flour mixture).

Add sugar, flour mixture, and cinnamon to a food processor and process to combine, about 20 seconds. Scatter butter over flour and process until mixture resembles a coarse cornmeal, about 10 seconds.

Add honey, milk, and vanilla; process until dough comes together. Transfer to a bowl, loosely cover and let rest 30 minutes.

Heat oven to 300°F with racks set to lower middle and upper middle position. Divide dough in half and form into 4×6-inch rectangles. Working with one rectangle at a time, roll between two pieces of parchment paper to ⅛-inch thick. Remove top sheet of parchment paper, trim dough into an even rectangle (save scraps), and cut into 2×2-inch squares. Prick each square a few times with a fork then transfer each dough-lined parchment paper to a baking sheet, chill 10 minutes.

Bake crackers until firm and golden brown, 30–32 minutes, rotating pans halfway through baking. Cool crackers completely on baking sheets then break apart. Repeat rolling, cutting, chilling and baking with scraps.