Bring water, sugar and corn syrup to a boil in a small saucepan. Reduce heat to low and gently simmer for 12 minutes.Meanwhile, seal candy canes in a zip-top plastic bag. Use bottom of a small saucepan to smash candy into tiny pieces. Place frozen yogurt in a food processor fitted with a metal blade. Add smashed candy canes and pulse to mix. Whisk grated chocolate and cocoa powder into thickened corn-syrup mixture until completely incorporated. Whisk in evaporated skim milk, vanilla and salt. Serve over peppermint-stick frozen yogurt. (If you're not serving immediately, you can transfer frozen yogurt to a lidded plastic container and freeze until ready to serve.)
One serving equals 1/3 cup yogurt and 2 1/3 tablespoons chocolate sauce.