Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners or grease the tin with cooking spray.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt until well combined.
In a separate bowl, whisk together the eggnog, vegetable oil, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Be careful not to overmix; a few lumps are okay. If using, fold in the chopped pecans.
In a small bowl, combine the melted butter, brown sugar, and ground cinnamon to prepare the topping. Mix until crumbly.
Divide the batter evenly into the prepared muffin tin, filling each cup about two-thirds full. Sprinkle the topping mixture generously over each muffin.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
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 with a dusting of powdered sugar and a sprinkle of extra cinnamon for a festive touch.