Preheat oven to 400 degrees F (200 degrees C). In the top of a double boiler, combine butter, sugar, 1/2 cup whole milk, cream. Heat until butter is melted and mixture is simmering, stirring occasionally. In a small bowl, combine 1/2 cup 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.