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