In a medium mixing bowl, whisk to combine the whole wheat and all-purpose flours, salt, cinnamon, baking soda, and baking powder.

In a large mixing bowl, combine the butter, brown and granulated sugars, honey, and vanilla. With a hand mixer (or in a stand mixer with the paddle attachment), beat for 2 to 3 minutes on medium-high speed until creamy and lightened in color. (Stop to scrape down the sides of the bowl as needed.)

Add the flour mixture in two batches, mixing on medium-low speed in between each addition. After the final addition, the mixture may look crumbly. Knead it together in the bowl with your hands, 5 to 6 times, until a cohesive ball forms. (If needed, drizzle in 1 to 3 teaspoons of the milk, then remix or knead the dough until it comes together.)

Turn the dough onto a large piece of plastic wrap and press it into a smooth, square shape, about ½-inch thick. Wrap it tightly with plastic wrap. Refrigerate it for 1 hour or up to overnight.

Preheat the oven to 300°F. Remove the dough from the refrigerator. (If refrigerated overnight, let the dough rest at room temperature for about 30 minutes or until softened. (It should be close to room temperature but still cool when ready to roll.)

Lightly flour a large sheet of parchment paper (12x16-inches). Place the dough in the center of the parchment paper and sprinkle flour lightly on top. Top with another sheet of parchment paper of the same size. Use a rolling pin to roll the dough between the two parchment sheets to be about ⅛ to 1/16th-inch thick. (If the dough starts to crack, it may be too cold. Let it warm up for 5 more minutes, then continue to roll.) Remove the top sheet of parchment paper.

Using a pizza cutter, or long knife alongside a ruler, trim the four borders of the dough to have flat edges. Cut the dough into about 2x3-inch rectangles (or any size you like) leaving the crackers touching, side-by-side on the parchment paper. Slide the whole sheet of parchment paper with the dough onto a half-sheet tray. Brush the crackers with a light coating of milk, being careful not to brush over the cuts. Dock (poke holes in) each cut graham cracker with the tines of a fork, 3 to 4 times, depending on how big they are cut.

Bake for 25 to 30 minutes, turning the pan 180° halfway through, until they are medium-dark brown in color and crispy. Let cool for about 5 minutes on the baking pan. At this point, they should be cool enough to handle, but warm enough to break cleanly along the pre-cut lines. Transfer to a wire rack to cool completely on a wire rack.