Position rack in top third of oven and preheat to 400°F. Using rubber spatula, scrape marshmallow creme into large bowl. Using electric mixer, beat egg whites and salt in another large bowl until foamy. Add sugar, 1 tablespoon at a time, and beat until stiff and glossy peaks form. Add 1/2 cup beaten egg whites to marshmallow creme and stir with rubber spatula or spoon just until incorporated to lighten (marshmallow creme is very sticky and will be difficult to blend at first, but blending will become easier as remaining whites are folded in). Fold in remaining whites in 2 additions just until incorporated. Spread meringue over top of cold pie, mounding slightly in center and swirling with knife to create peaks.
Bake pie just until peaks and ridges of marshmallow meringue are lightly browned, about 4 minutes. Let stand at room temperature until meringue is cool. (this is particularly good on a Sweet Potato Pie!)