Preheat oven to 400 degrees F (200 degrees C).
In the top of a double boiler, combine the butter, sugar, ½ cup whole milk, and whipping cream. Heat until butter is melted and mixture is simmering, stirring occasionally.
In a small bowl, combine ½ cup cold whole milk and cornstarch. Whisk until smooth, then slowly pour into hot cream mixture, whisking constantly. Simmer mixture until thickened, about 5 minutes, stirring constantly. Pour into baked pastry shell and sprinkle with ground nutmeg.
Bake in preheated oven for 5 minutes. Allow to cool before serving.
