How to Make It
In a bowl, beat 4 tablespoons of the butter until creamy. Stir in the scallion, lemon juice and mustard until smooth.
Preheat a griddle over low heat. Spread the scallion butter on the cut sides of the rolls. Lay the prosciutto on the bottom halves; top with the Brie and the apple slices and close the sandwiches. Lightly spread the remaining 2 teaspoons of butter on the outside of the rolls (it will be a very thin layer).
Put the sandwiches on the griddle. Cover with a heavy skillet and cook over low heat, turning occasionally, until toasted and the Brie is melted, 10 minutes. Cut the panini in half and serve.