Step 1: In a small saucepan over medium heat, simmer 2 cups of apple cider until it has reduced to 1/2 cup (about 20 minutes). Set aside to cool.
Step 2: Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease with non-stick spray.
Step 3: In a large bowl, whisk together the flour, dark brown sugar, baking powder, baking soda, cinnamon, nutmeg, and salt, ensuring there are no large lumps of sugar.
Step 4: In a separate medium bowl, whisk together the reduced (and cooled) apple cider, vegetable oil, egg, buttermilk, and vanilla extract.
Step 5: Gently fold the wet ingredients into the dry ingredients using a spatula. Mix just until combined; do not overmix.
Step 6: Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Step 7: Mix the 2 tablespoons of granulated sugar and 1/2 teaspoon of cinnamon in a small dish. Sprinkle this mixture generously over the top of each muffin.
Step 8: Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Step 9: Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.