Bring about 2 inches of water to a simmer in a medium pot. Meanwhile, place the cool water in the work bowl of a stand mixer and sprinkle in the gelatin. Wait 5 minutes, then whisk in the corn syrup and the salt. Set the bowl over the simmering water and stir to dissolve the gelatin.
Remove the bowl from the heat and install on the mixer. Using the paddle attachment, slowly beat the powdered sugar into the gelatin mixture 1 cup at a time until you have a stiff dough the consistency of play-dough. You may not need all of the powdered sugar, but you can use leftovers for rolling.
Turn the dough out onto a powdered sugar-dusted counter and knead until smooth. Divide dough into 6 portions (about 8 ounces each) and keep wrapped in plastic wrap until ready to work.
Make a well in a dough portion and place 2 teaspoons of the flavored powdered gelatin of your choice and 1/2 teaspoon of water into the well. Fold the dough in and knead. The dough may seem sticky or crumbly when you first add the gelatin, but keep kneading and it will smooth out. Feel free to taste and add more gelatin as needed to increase the flavor or color. Repeat with each 8-ounce portion. Be sure to wrap up each portion as you work with it so that it doesn't dry out.
If you're using food coloring and flavored oils or bitters, start with 2 drops of food coloring and 1/2 teaspoon oils or bitters. I prefer to work in the flavors then the coloring.
Lightly sprinkle a sheet of plastic or parchment paper with powdered sugar, then flatten out a portion of dough and sprinkle with additional powdered sugar to ensure against sticking. Roll the dough out to a 3/8-inch sheet.
You can easily find a heart-shaped cookie cutter via interwebs, but if you don't have one in your possession, simply use a small circle cutter or even an apple corer to cut out rounds. Shape the bottom of the heart into a point by pinching with the fingers and carve the cleft at the top with a channel knife or paring knife.
Arrange the hearts on parchment-lined sheet pans to air-dry for 48 hours, flipping after the first 24.
Once dry, use the edible ink pen (interwebs again) to adorn the hearts with your wit, wisdom, sentiments, and sarcasm. Then, simply store in an airtight container until you serve them up to the object of your obsession...I mean, affection.