Bring milk to a simmer in a 4-quart heavy saucepan and transfer 1 cup to a blender.
Add malted milk powder and brown sugar to remaining hot milk, whisking until dissolved.
Remove from heat and stir in vanilla and rum (don't forget the rum).
Blend milk in blender at high speed until foamy, about 1 minute.
Divide malted milk among 4 mugs, then top with foamy milk and sprinkle with nutmeg.