Preheat oven and line pan:

Heat oven to 350°F (175°C). Insert paper liners into a standard 12-cup muffin pan.

Whisk dry ingredients together:

In a medium bowl, combine 2 cups flour, granulated sugar, baking powder, 1 tsp cinnamon, ½ tsp salt, and nutmeg with a whisk.

Mix wet ingredients:

In another bowl, whisk together 1 cup eggnog, eggs, ¼ cup melted butter, and vanilla.

Fold dry into wet:

Gradually fold the dry mixture into the wet mixture until barely combined.

Fill muffin cups:

Divide the batter evenly among the lined cups, filling each generously but not to the brim.

Combine streusel dry mix:

In a medium bowl, stir together 1 cup flour, brown sugar, 1 tsp cinnamon, and ½ tsp salt.

Cut in melted butter:

Pour in ½ cup melted butter and blend with a fork until crumbly.

Top muffins with streusel:

Top each muffin with about 1 tablespoon of the streusel.

Bake until done:

Bake for 22-24 minutes, until a toothpick inserted into the center comes out mostly clean with a few moist crumbs.

Cool muffins completely:

Let muffins cool completely on a wire rack.

Mix eggnog glaze:

In a small bowl, mix confectioners' sugar with 2 tablespoons eggnog until smooth. Adjust with more eggnog if needed.

Drizzle glaze over muffins:

Drizzle the glaze over the cooled muffins.