Here are 2 different versions, I prefer the first recipe personally as it most closely resembles the store bought sugar free syrup in taste and texture.
1 1/3 cups Splenda
1 dash salt
1 1/2 cups water
1 tsp vanilla extract
Mix the cocoa, splenda and salt in a saucepan until mixed, add water. Bring to a boil over medium heat until sauce starts to thicken. Remove from heat add vanilla extract. Cool and store in the fridge.
1/3 c Dry cocoa
1 1/4 c Cold water
1/4 tsp Salt
2 tsp Vanilla
3 tsp Liquid Sucaryl (or other sugar replacement equivalent)
Combine all the ingredients together, stirring well. Use as you would any chocolate syrup but be sure to store this in the refrigerator.