Place gelatin and sugar in a bowl. Add boiling water to gelatin mixture; stir 2 minutes to completely dissolve. Refrigerate until very thick, about 30 minutes.
Meanwhile, place milk and mixer beaters in a bowl. Freeze until ice crystals form around edge of bowl, about 30 minutes. Beat milk on high speed until light and fluffy. While beating, pour in thickened jello mixture; beat on high until soft peaks form.
Pour into an 11x7-in. dish. Mix cracker crumbs and butter; sprinkle over top. Refrigerate, covered, at least 4 hours before serving.