Preheat oven to 350F. Line a 12-hole muffin tin with paper liners, or spray the muffin holes with non-stick spray.
In a mixer fitted with a paddle attachment, beat the butter, sugar and vanilla extract until light and creamy. Add the eggs one at a time and beat until incorporated after each addition.
In a separate bowl, whisk together the flour, baking powder, baking soda and nutmeg. With the mixer on low, add ? of the flour mixture and beat until well blended. Add ½ of the milk and beat until incorporated. Repeat, with the remaining flour and milk, beating well after each addition, ending with the last ? of the flour. Mix just until incorporated.
Divide batter evenly among muffin liners, filling each around ? full. Bake 15-20 minutes, or until the tops spring back when lightly touched and are light golden.
Combine topping ingredients in a smallish bowl with a tight-fitting lid. I find the easiest way to combine the sticky vanilla beans into the sugar is to put the lid on the bowl and shake, shake, shake!
Brush the tops of the warm muffins with the melted butter and liberally sprinkle over the topping mixture.
Enjoy warm from oven! They will keep in an airtight container at room temperature for a day or so.