Note: Reduce the amount of water by 50% to make firmer jelly cubes.
Dissolve each box of Jell-O with a cup of boiling water in separate shallow containers to make cubes no larger than an inch. Stir until the powder completely dissolves.
Refrigerate for about an hour until solid. Cover them so they don't pick up other smells.
Once they are solid, cut them into small cubes (about ¾ inch). Set them aside.
Dissolve the contents of 4 clear, unflavored envelopes with 2 cups of boiling water. Keep whisking as you pour the water into the bowl to dissolve the powder and eliminate the need to "bloom" the gelatin.
Add condensed milk and heavy cream. Whisk until well-blended.
If you like a sweeter mixture, add up to ¼ cup of granulated sugar at this point.
Gently arrange the jello cubes in a dish in any pattern you like.
Pour the cream gelatin mixture over the jelly cubes. The mixture shouldn't be too warm, or it could melt the jelly cubes.
Cover and refrigerate for about 2 hours, or until solid.
Video
Notes
Jell-O: You will need 5 (3-ounce) assorted flavors of Jell-O or "gelatin dessert" to make a total of 5 cups. I used grape, berry, orange, lime, and cherry flavors. Agar-agar or gulaman, a plant-based gelatin, is an excellent substitute.
Unflavored gelatin: I used 4 envelopes (0.25-ounce or 2 ½ teaspoons) of Knox gelatin — clear, unflavored granulated gelatin.
Heavy cream: You may also use all-purpose cream, light cream, or half-and-half. Keep in mind that the less fat in it, the softer it will be.
Sugar: As an optional ingredient, you can add up to ¼ cup of granulated sugar to the cream gelatin mixture.