In a medium-large mixing bowl, pour cold (refrigerated) juice, and whisk in the magnesium powder slowly. It will foam up quite a bit—so whisk in a tablespoon, then gave it a few minutes for the foam to die down. Whisk in another tablespoon, gave it another few minutes, etc., until it was all in there, with minimal foam.
In a small saucepan, pour in the juice/magnesium mixture. Now sprinkle the gelatin powder over the juice mixture in your saucepan, but leave it off the burner. We don’t need any heat yet. Allow it to sit for a couple minutes in order for the gelatin to “bloom”—this is a critical process of the gelatin powder absorbing the liquid. Without proper blooming, you will have a very icky, gritty texture to your gummies, and you will not be able to fully dissolve the gelatin once you heat your mixture. Very important to bloom! After a few minutes, you’ll see the mixture start to gel and form a weird, wrinkly mass. This is good! If there are still any bits of white powder sitting on top of the wrinkles, whisk them in while the mixture is still cold. When there’s no more white powder or visible white clumps, you’re okay to move onto the next step.
Now, you can go ahead and turn on the burner to a low-medium temp. As your mixture heats up, it will begin to liquify. Once it does, add in sugar and salt to dissolve. Avoid whisking it too much, or it’ll start to foam up just like it did when you added the magnesium powder. If it does, that’s okay, just let the foam sit for a few minutes and it’ll calm down.
Alright. Now, get yourself a glass liquid measuring cup with a spout, and pour the liquid gummy mixture into the measuring cup. Scrape the sides of the pan with your spatula to get it all in there. Is it 3/4 of a cup of liquid still? If it is, then it’ll fit exactly into the silicone mold I used, if you want to use that, too. And if it isn’t, that’s probably because some of the liquid evaporated, so you can just add in a little more juice if you’d like.
Set the silicone mold onto a large plate or a cutting board—any kind of hard and portable surface so you can get it into your fridge without spilling, since the silicone is soft. Pour the mixture very slowly and carefully into the silicone mold. If you’re not using a mold, you can pour it into a small baking dish instead, and then you’ll be able to cut out squares after it’s set.
Refrigerate your molds at least a half-hour, or until fully set. Then, carefully peel/wriggle/pop them out of the molds, or slice into squares if poured into a baking pan.