Preheat the oven to 350°F (175°C).
Cream the butter and brown sugar in a large mixing bowl until light and fluffy.
Mix in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the whole wheat flour, all-purpose flour, cocoa powder, baking soda, and salt. Gradually add to the butter mixture until a dough forms and pulls away from the sides of the bowl.
Roll out the dough to ¼ inch or ⅛ inch thickness between two sheets of parchment paper.
Rolling between parchment makes it easier and prevents sticking.
Use a fluted cookie cutter to create uniform squares, or score with a pizza cutter, and prick with a fork for a traditional graham cracker look.
Bake on a parchment-lined cookie sheet for 15 minutes (for ¼ inch) or 10-12 minutes (for ⅛ inch), until crisp.