Pour the seltzer into a small saucepan. Sprinkle the gelatin over the liquid and allow it to soften, about 5 minutes.
Turn the heat on to medium-low and, while stirring constantly, dissolve the gelatin completely. Do not let boil. Remove from heat. Stir in the champagne and vodka.
Transfer mixture to a container with a spout, like a glass measuring cup. Pour mixture into prepared vessels and allow to chill in the refrigerator 3 to 4 hours before serving.
To serve: use a small spatula or butter knife to break the seal between the shot and the silicone mold. You may also dip the bottom of the mold into hot water for a few seconds to help loosen. Flip gelatin mold over onto serving tray or pop them out individually. Store leftovers in the refrigerator.
Gelatin: I did not want my shots to be overly sweet, so I went with unflavored gelatin. If you want to amp up the flavor or sweetness, use 1 and ¾ Tablespoons of flavored, sweetened gelatin (like Jell-O).
Champagne: I chose a sweet Rosé champagne.
Vodka: I chose plain, unflavored vodka. You may use flavored, if desired.
Please keep in mind, as written and without substitutions, your shots will taste exactly the way your champagne tastes (so as sweet or as dry as the one you choose). With unsweetened but flavored seltzer, unflavored gelatin, and unflavored vodka, our shots tasted just like the sweeter champagne we chose and they were perfect for us.