According to GourmetSleuth.com, a mixture of marshmallows and corn syrup can be used as a substitute for marshmallow creme. Another option is making marshmallow creme from scratch with egg whites, sugar, water, vanilla and salt.
The quickest substitute for marshmallow creme is made with marshmallows. Combine 7 cups of miniature marshmallows and 2 teaspoons of corn syrup in a double boiler. Continuously heat and stir the mixture until the marshmallows are completely melted. Make homemade marshmallow creme by heating 3/4 cup of sugar and 1/4 cup of water together. Combine 4 egg whites, 1 teaspoon vanilla and a pinch of salt in a bowl. Whip until soft meringue forms. Gradually add 1/4 cup of sugar to the mixture. Continue whipping the meringue until medium peaks form. Add slightly-cooled sugar and water syrup to the meringue. Continue whipping until stiff peaks form in the meringue.