Preheat your oven to 350°F (175°C). Prepare a muffin tin by lining it with paper liners or lightly greasing it with non-stick cooking spray.
In a large mixing bowl, combine the all-purpose flour, granulated sugar, packed brown sugar, baking powder, baking soda, salt, and ground cinnamon. Whisk these dry ingredients together until thoroughly mixed.
In a separate bowl, crack the large egg and beat it gently. Add the buttermilk, vegetable oil, and orange zest, whisking until well integrated.
Pour the wet mixture into the bowl of dry ingredients. Stir gently until just mixed, leaving a few small lumps.
Carefully fold in the chopped fresh cranberries and chopped walnuts or pecans (if using).
For the crumb topping, combine the flour, packed brown sugar, softened butter, and ground cinnamon in a small bowl. Mix until it resembles coarse crumbs.
Divide the muffin batter evenly among the prepared muffin cups, filling each about three-quarters full. Sprinkle the crumb topping over each muffin.
Bake the muffins for 18-20 minutes until a toothpick inserted comes out clean or with a few moist crumbs.
Allow the muffins to cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.
Notes
Serve warm for the best flavor. Optionally dust with powdered sugar or add whipped cream.