In a pan or a skillet over medium heat, layer the brown sugar and let it melt.
Stir occasionally as the sugar continues to melt and gets to this light brown color.
Continue stirring until the color deepens and looks dark brown like this. Now take this off heat.
To this, add the hot water and stir until it gets to a uniform consistency. Allow it to cool down, then transfer into a jar or a container.
Notes
Resist the urge to turn up the heat. The sugar can quickly turn black and eventually, the syrup will end up tasting bitter.
Make sure you don't walk away from the pan once the sugar has started to brown up. The timing is crucial, or you'll end up with very dark, almost black sauce that doesn't even taste great.
Be careful while pouring the hot water into the sugar mixture. Avoid any hot splatters on you.
Make sure you use boiling hot water for this recipe. If the water has cooled down, it'll cause the sauce to become lumpy.