Preheat oven to 350°F. Line a muffin pan with paper liners or spray with cooking spray. Set aside.
To Make The Streusel Topping
In a small bowl, combine flour, brown sugar, cinnamon and nutmeg. Cut the butter Into small pieces and add to the flour mixture. Rub the butter into the flour mixture until it resembles coarse breadcrumbs. Set aside.
To Make The Muffins
In a large bowl, whisk together flour, sugar, baking powder, cinnamon, nutmeg and salt. Set aside.
In a medium bowl, whisk together the eggnog, egg, melted butter and vanilla.
Pour the wet ingredients Into the dry ingredients. Stir until just combined. Do not overmix.
Fill each muffin cup about ⅔ of the way full. Top each muffin with streusel (about 2 teaspoons worth).
Bake muffins for 30 - 40 minutes, or until a toothpick inserted in the center comes out clean.
Allow the muffins to cool In the pan for 10 minutes, then remove to fully cool on a wire rack.
To Make The Eggnog Glaze
In a medium bowl, whisk together the powdered sugar, eggnog and pinch of nutmeg. Whisk until smooth.
Once muffins are completely cooled, drizzle the glaze over each one.