For the infused peppermint simple syrup: Combine the crushed candies with 1 1/2 cups of water over medium heat. Simmer until the candy is dissolved; remove from the heat. Strain if any candy bits remain. Let cool.
For the chocolate peppermint sodas: Pour the crushed candies into a small dish. Dip the rims of 2 highball glasses in water and roll in the crushed candies to coat the rims. (You may need to press them on a bit with your fingers to evenly coat the rims.)
Fill the glasses halfway with ice. Add 1 ounce of the simple syrup to each glass, divide the seltzer between the glasses and stir to mix. Drizzle in the chocolate syrup and let it swirl down into the pink soda. Add a splash of vodka if desired.