In a high-sided saucepan over medium heat, bring sugar, water and corn syrup to a boil, stirring the mixture until sugar dissolves.
Attach a candy thermometer to the side of the pan, making sure that the tip is submerged but does not touch the bottom of the pan. Don't stir the mixture again once the sugar is dissolved. Wait for the candy thermometer to reach 290°F to 300°F which is the hard crack stage of candy making. This will take some time, so prepare to be patient. If you crank the heat the mixture will turn a darker shade of amber. It’s up to you.
If you don’t have a candy thermometer, cook until mixture turns pale gold. Test it by dipping a spoon in the syrup. Wait until it cools then carefully touch the candy on the spoon. If it’s sticky, you’re not there yet! It should be hard, like glass.
Be very careful as the heated sugar mixture is extremely hot and will stick like lava to skin if spilled. Always use hand protection when dealing with heated sugar syrup.
Remove from heat, add flavoring if using, and immediately pour syrup onto rimmed baking sheet. Working quickly, tilt (the HOT) pan to spread candy to edges to make a very thin layer.
Let cool at room temperature to harden. Depending on temperature and humidity, this could take up to an hour.
Twist pan to release and break the candy. Use a metal spoon, or other object, in order for it to resemble broken glass.
Place a shard or two of sugar glass in the center of each cupcake. For added effect, drizzle or pipe red tinted preserves where candy glass has entered the cupcake.