In the bowl of a standing mixer fitted with the paddle attachment or a large mixing bowl, stir together the all-purpose flour, white whole wheat flour, vital wheat gluten, yeast, and salt. Add the butter, egg, and orange zest.
Heat the buttermilk and orange juice to 120-130°F. If the buttermilk separates, simply whisk it back together. Pour the liquid into the dry ingredients, mix on low speed (or mix by hand with a wooden spoon) until the ingredients come together and form a ball.
Switch to a dough hook (or turn out onto a lightly floured surface) and mix on medium speed for 8 minutes or knead by hand for approximately 10 minutes, until the dough is soft, pliable, and slightly tacky but not sticky. Place the dough in a lightly oiled large bowl, cover with plastic wrap, and let rise at room temperature for approximately 2 hours, until the dough doubles in size. To check dough readiness, gently press the dough with two fingers. If the indent stays, the dough is ready.
Prepare the cranberry filling: In a food processor, pulse the whole cranberries until they’re ground and pebbly, but not completely pureed, scraping the machine down once or twice. In a small bowl, combine the brown sugar and cinnamon. Set both aside.
Lightly oil two 8x4-inch loaf pans. Divide the dough into 2 equal pieces (use a kitchen scale if you have one). Place one piece on a lightly floured work surface and pat into a rectangle that is 5 inches wide by 8 inches long. Brush with half of the melted butter, sprinkle with half of the cinnamon sugar mixture (it will seem like a lot, but pile it thickly), and scatter half of the cranberries over the top.
Working from the short (5-inch) side of the dough, roll up the length of the dough, jelly-roll style, pinching the crease with each rotation to seal in the filling and strengthen the dough’s surface tension. The loaf will spread as you roll, reaching 8 to 9 inches (the length of your loaf pan). Pinch the final seam closed, then lift the loaf and rock it gently back and forth to even it out. Place the loaf in one of the prepared pans. Repeat with second piece of dough. Mist two pieces of plastic wrap with cooking spray and loosely cover the pans. Let the loaves rise at room temperature for 60 to 90 minutes, until the dough is nearly doubled in size. (Alternatively, you can let the loaves rise in the refrigerator overnight, then bake the next morning, allowing the loaves to rest at room temperature for 30 minutes before baking.)
Place rack in center of oven, then preheat to 350°F. Place the loaf pans on a large sheet pan, ensuring that they do not touch. Bake for 20 minutes, rotate the sheet pan 180 degrees, then bake an additional 20 to 30 minutes, until the loaves reach an internal temperature of 190°F. The finished loaves will be brown on the top, golden on the sides, and sound hollow when tapped.
Immediately remove the bread from the pans, then let cool for at least 1 hour before serving. Enjoy warm with butter.