Edible sugar diamonds add sparkle and shine to a cake. Making your own requires just a few ingredients, but a beginner may need several practice runs before completing a successful batch. Read and understand all instructions before you start, and make sure you have the right kitchen equipment and protection. The hot sugar mixture can cause severe burns, so wearing gloves, an apron and other protective clothing is essential.
Put a large, tempered-glass measuring cup in the center rack of a cold oven. Turn on the oven to 275 degrees Fahrenheit and leave the cup inside to heat up with the oven.
Whisk ½ teaspoon of cream of tartar for every cup of white granulated sugar in a bowl until thoroughly mixed. Start with 2 cups of sugar for a small batch. Add the cream of tartar to stabilize the sugar.
Pour the sugar mixture into a clean stainless steel saucepan. Add half as much water as the sugar mixture in the saucepan. For example, for 2 cups of the sugar mixture, add 1 cup of water. Add 2/3 cup of light corn syrup for every cup of water. Turn on the heat and bring the mixture to a boil.
Put on heat-proof gloves and wear other protective clothing, because the hot sugar mixture can cause third-degree burns. Carefully attach a candy thermometer to the saucepan with the probe suspended in the mixture only after it starts to boil. If you attach the thermometer before it boils, the sugar will crystallize around the thermometer, and crystallization can ruin the batch.
Dip a silicone or pastry brush in water and brush any crystallized sugar from the sides of the saucepan when the sugar mixture starts to boil, again wearing heat-proof gloves. Sweep the crystals from the sides downward and back into the boiling sugar mixture. Keep dipping the brush in water after every two or three strokes. After you wash down the sides of the pan, do not stir the mixture again.
Watch the thermometer until the mixture reaches 300 degrees Fahrenheit. Do not leave the pan unattended, even though it may take 20 minutes or longer to reach that temperature. Keep a close eye on the thermometer after it hits about 270 F, because the temperature will begin to rise much more quickly. Do not allow it to rise above 300 degrees Fahrenheit or the sugar will burn.
Take the saucepan off the heat when the temperature reaches 300 F and place the pan on a cold burner of the stove. Let the pan sit until the temperature falls to 275 F; it may rise a bit before it falls. Do not move, stir or do anything else to the mixture during this time.
Wear your gloves and take the hot glass measuring cup out of the oven. Pour the sugar mixture into it. Do not scrape the pan; just let the mixture flow into the cup. Do not pour the hot sugar mixture into a cold measuring cup, because cold glass may break from thermal shock. Return the measuring cup to the oven to sit for five to 10 minutes to allow the air bubbles to rise to the top and escape while the sugar remains hot enough to flow.
Position a silicone kitchen mat on your workspace to protect it. Spray candy diamond molds with cooking spray. If you are using silicone molds, you do not need to spray them with cooking spray.
Wear the gloves and remove the cup of sugar mixture from the oven. Carefully ladle small amounts of the hot sugar mixture into the diamond wells of the molds with an extremely clean spoon. Push any drips between diamond wells into the nearest well.
Allow the sugar to cool completely, probably about 15 minutes. Turn the diamond molds over and flex them slightly to release the finished edible sugar diamonds onto the silicone mat.