In a medium bowl, whisk together the whole wheat flour, all-purpose flour, brown sugar, cinnamon, baking soda, and salt.
Add the cold, cubed butter and use a pastry cutter or your fingertips to work it into the flour mixture until it looks like coarse crumbs.
Stir in the oil, honey, molasses, vanilla, and 3 tablespoons of milk. Mix until a dough forms. If it seems dry, add the remaining milk 1 teaspoon at a time until the dough holds together.
Form the dough into a disk, wrap it in plastic wrap, and refrigerate for 30 minutes.
When ready, preheat oven to 325°F. Line two baking sheets with parchment paper.
On a lightly floured surface, roll the dough very thin, about ⅛ inch thick.
Cut into squares or rectangles, about 2 ½ × 2 ½ inches. A pizza cutter works well. Transfer them to the prepared baking sheets.
Prick each cracker several times with a fork or skewer to prevent bubbles during baking.
Lightly sprinkle the crackers with a cinnamon-sugar mixture (if using).
Bake 10 to 14 minutes, until lightly golden and dry to the touch. Rotate the pans halfway through for even baking. (Keep an eye on them! Depending on the thickness, they could be done sooner.)
Let crackers cool completely on a wire rack. They will crisp further as they cool.
Keep crackers in an airtight container at room temperature for up to 1 week, or crush into homemade graham cracker crumbs for pie crusts or desserts.