Stained Glass Jello Cake! .. Make the jello squares 1 box jello to 1 cup boiling water. Chill, then cube it. Then In a separate bowl, sprinkle 2 envelopes unflavored gelatin into 1/2 cup cold water. After the gelatin blooms, add 1 1/2 cup boiling water and dissolve. Add can of sweetened condensed milk. Stir and cool. Pour cooled milk mixture over jello and chill overnight.