Combine marshmallows, salt, and milk in top of a double boiler. Bring water in bottom of double boiler to a boil. Cook over boiling water, stirring frequently, until marshmallows melt and mixture is smooth. Cool 15 minutes. Add peanut brittle; stir well.
Carefully fold whipped cream into peanut brittle mixture; pour into graham cracker piecrust. Chill until firm.