Preheat oven to 350. Grease 4 cups in a jumbo muffin pan. Fill the 2 empty cups halfway with water.
In a large bowl stir together sugars. Add flour and nutmeg, whisking until blended. Cut butter into flour/sugar mixture with a fork until crumbly. Remove and reserve 1/2 cup crumb mixture for topping.
Combine buttermilk, vanilla, baking powder, baking soda and salt. Whisk in egg. Pour over the crumb mixture remaining in large bowl. Stir with a fork until dry ingredients are moistened.
Spoon batter evenly into a greased jumbo muffin pan, filling 4 muffins with batter. Sprinkle reserved crumb mixture evenly over batter, pressing lightly into batter.
Bake 350 for 28 to 30 minutes or until a wooden toothpick inserted in the center comes out clean. Cool in pan on a wire rack for 10 minutes, remove cakes from pan, and let cool completely on wire rack. Drizzle with Vanilla Glaze.
Vanilla Glaze: Whisk all ingredients until smooth. Yield 1/4 cup.