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Preheat the oven to 300°F with racks in the upper and lower thirds. (You'll be rolling, transferring, and baking the graham crackers on parchment, so no need to grease any baking sheets.)
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In a large bowl, whisk together the flour, sugar, baking powder, cinnamon, and salt.
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In a medium bowl, whisk together the starter, oil, and egg.
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Add the wet ingredients to the dry ingredients and stir to form a cohesive dough.
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Divide the dough roughly in half (about 270g each) in the bowl. and shape each half into a flattened square.
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Working with one piece at a time, transfer the dough to a sheet of lightly greased parchment and shape into a flattened square. Place another piece of lightly greased parchment over top and roll the dough to about 1/16" thick; aim to cover the majority of the parchment, leaving a 1" to 2" border. Try to keep the dough a uniform thinness so the crackers bake evenly.
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Use the parchment to transfer the sheet of dough to a baking sheet, then gently peel off the top sheet. Repeat with the remaining piece of dough.
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Generously sprinkle the dough with cinnamon sugar, if desired.
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Bake the sheets of dough for 10 minutes. Remove them from the oven and use a pizza wheel or sharp knife to trim a thin border around each, creating clean edges. Cut each sheet into approximately 3" x 2" crackers; don't separate them yet. For that distinctive graham cracker look, you can use a fork to prick the crackers at this point.
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Return the crackers to the oven and continue to bake for 20 to 25 minutes, rotating the baking sheets top to bottom and front to back halfway through, until starting to brown around the edges.
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Turn off the oven and let it cool down completely with the crackers inside, at least 2 hours and up to overnight; this will help the crackers become as crisp as possible.
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Store the cooled sourdough graham crackers in an airtight container at room temperature for several weeks.
