Wash the sticky rice once or twice to remove any dust or dirt, the water does not need to come out completely clear. Then soak the rice in 200 ml of water for at least 4 hours up to overnight, we will be using the water that the rice is soaked in so use drinking water;
In a nonstick pan, dry toast the sesame seeds over low heat until you can smell the aroma, stir and flip frequently, this process will take about 5, do not burn them!! Otherwise your soup will be bitter. And do not skip this step, toasting first is the only way to fully release all the amazing aroma;
When the sesame is done toasting, let them cool down, then transfer the soaked sticky rice plus the water the rice had been soaking in, sesame seeds, and rest of the water (300ml) to a blender, you can add more or less water depending on your preference of thickness, the amount of water is flexible here;
Blend on high speed until smooth, if it looks too grainy, give it a shake and blend again, you'll achieve better result with a powerful high-speed blender;
Pour the blended mixture over a fine-mesh strainer into a nonstick pot, if it's blended properly, there should be barely any scraps left on the strainer;
Add brown sugar or sweetener of choice to the pot, cook over low heat, keep stirring do not step aside at this point! The soup can easily burn at the bottom if not being stirred. you will start to see the mixture thickening, and the color will darken as well, from a gray color to almost black, it's ready to serve at this point, but if you are add the mochi, then take it off heat, cover and keep the soup warm;
To make the mochi, add rice flour and sugar to a bowl, gradually add the water into the dry ingredients while stirring with the other hand;
Then knead till a smooth dough forms, the dough should be soft but not sticky, different brands of glutinous rice four has different levels of moisture in them, so if the dough feels too wet, add a touch more flour, if it's too dry, wet your hands then continue to knead more moisture in;
Cover and let the dough rest for 30 minutes;
When the dough is rested, roll it into a thin cylinder shape, then cut into bite-size, you can either use a mold or channel your childhood self and shape them with your hands like play dough, let your whimsy run wild here any shape you like!
Bring a pot of water to boil, gently drop the mochi inside, cook till they are floating on the surface, strain them out and drop them in an ice water bath to cool down;
Serve the black sesame soup warm or cold, it's good both ways, with tang yuan on top, enjoy!