Heat the water and sweetener in a metal-bottom pan until just simmering, then lightly stir the water until everything dissolves.
Once your sugar water is gently simmering, add your pinch of saffron, stirring them in until each thread is wet. If using saffron water, don't add it until you turn off the heat.
Heat the saffron simple syrup mixture on low for 4-5 minutes (set a timer), never letting it get above a simmer, stirring only if the saffron starts to all rise to the top.
When your timer goes off, remove the mixture from the heat and let everything sit on a cool burner for 20-30 more minutes (to infuse further). This is when you should add any rose water or vanilla, or saffron syrup (if using).
Finally, once the timer goes off, strain the mixture and preserve your saffron simple syrup in a closed container in the fridge for up to 10 days or in the freezer for up to 6 months.