Heat oven to 375 degrees F. Line muffin pan with 12 standard or 6 large paper cupcake liners; set aside.
In large bowl, stir together 2 cups flour, baking powder, 1 teaspoon cinnamon, baking soda, salt, and nutmeg.
In small bowl, whisk together egg, buttermilk, pumpkin, 2/3 cup brown sugar and 1/3 cup melted butter until well mixed.
Add pumpkin mixture to flour mixture; stir just until blended, scraping sides of bowl.
Spoon batter into prepared muffin pan. In small bowl, combine all streusel ingredients until crumbly. Crumble mixture evenly over muffin batter in pans.
Bake at 375 degrees F. 18-20 minutes for standard sized muffins; 20-25 minutes for large muffins. Muffins are done when toothpick inserted comes out clean. Remove from oven and allow to cool 5 minutes in pan. Remove from pan and cool 10 minutes.
While muffins are cooling, in small bowl combine powdered sugar and maple syrup. Stir in water, adding enough until just thin enough to drizzle. Drizzle icing over slightly cooled muffins. Serve warm.
Makes 12 standard or 6 large muffins.