In a food processor, pulse flour, cocoa powder, graham flour, brown sugar, baking soda and salt to combine.
Add butter and pulse until pieces are the size of small peas.
Whisk honey, milk and vanilla in a small bowl.
With the processor running, slowly pour in honey mixture and process just until dough forms.
Divide dough in half, flatten into discs, wrap and refrigerate for at least 30 minutes.
Preheat the oven to 350°F and line 2 baking sheets with parchment paper.
Roll dough to 1/8-inch thickness on a cocoa-dusted surface.
Cut into 2-inch squares and transfer to baking sheets.
Prick with a fork.
Bake for 10 minutes, sprinkling with sugar halfway through if desired.
Cool completely before storing.