Crunchy, ultra-light zwieback toast has been a favorite of teething babies for years. Add a layer of cinnamon sugar to the toasts as they bake, and you've made Trenary Toast, a specialty of the Trenary Home Bakery in Michigan's Upper Peninsula. The cinnamon version of zwieback also mimics Finnish korpu, if you're partial to those tasty toasts.
Combine all of the dough ingredients (everything except the cinnamon sugar), and mix and knead to form a smooth, soft, somewhat sticky dough.
Cover the dough, and allow it to rise till it's quite puffy, about 2 hours.
Divide the dough in half. Each half will weigh just under 16 ounces.
Shape each half into a 12" log, and place on a lightly greased or parchment-lined baking sheet. Cover the logs, and allow them to rise for about 90 minutes, till they've expanded (though they won't have doubled in bulk). Towards the end of the rising time, preheat the oven to 350°F.
Bake the bread for 22 to 24 minutes, till it's a deep, golden brown, and its temperature at the center is at least 190°F.
Remove the bread from the oven, and allow it to cool on a rack. Let it rest at room temperature overnight; don't cover it.
Next day, cut the loaves into 1/2" slices.
Lay the slices on two ungreased or parchment-lined baking sheets.
Bake the slices in a preheated 200°F to 225°F oven for 1 hour. Remove the pans from the oven, and turn all the pieces over. For cinnamon toasts, sprinkle about 1/4 teaspoon cinnamon sugar evenly over the top surface of each toast.
Return the pan to the oven, and bake for an additional 1 hour, or until the toasts are very dry and crisp.
Remove from the oven, and cool completely. Store, well-wrapped, at room temperature.
