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Combine the whole wheat flour, all-purpose flour, sugar, salt, cinnamon, and baking powder in a medium-sized bowl.
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In a separate bowl, whisk the egg with the oil, honey, and 2 tablespoons milk. Stir this egg mixture into the dry ingredients until you have a fairly stiff dough, adding more milk if necessary. Knead the dough gently until smooth.
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Wrap the dough and chill it until firm, about 1 hour (or longer, if it's more convenient).
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Preheat the oven to 300°F.
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Divide the dough in half and, working with one piece at a time, roll the dough out about 1/16" thick onto a piece of parchment paper.
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Transfer the rolled-out dough on the parchment paper to a baking sheet. Repeat with the second piece of dough.
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Brush both pieces of dough with milk then sprinkle with the cinnamon sugar.
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Bake the sheets of dough for 10 minutes, rotating the pans after 5 minutes.
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Remove pans from the oven, and use a rolling pizza wheel or sharp knife to cut the sheets of dough into 3" x 2" rectangles; don't separate them, just cut them.
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Return the cut crackers to the oven, and continue to bake for 18 to 20 minutes.
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Turn off the oven, and open the oven door wide for 5 minutes. After the majority of the oven's heat has dissipated, shut the oven door, and let it cool down for 20 minutes with the crackers inside; this will help them become as crisp as possible.
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Remove the crackers from the oven, transfer them to a cooling rack, and cool completely.
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Store the crackers, well-wrapped, at room temperature for up to a week; freeze for longer storage.