Nowadays, you can easily get black sesame seed powder from shops, but I rather make my own to ensure I get them toasted the way I like it. It's not that hard.
Black sesame seeds are black. Yeah.
You can't see well if they are black. Whether they are over toasted or undertoasted.
To counter that, add in some white sesame seeds. Not too much white sesame seeds so that the powder stays as black as possible.
Some will choose to wash them before use, for me, I prefer to sift them.
I put the seeds into a sieve, that is not too fine. And give it a few shakes. Some stuff will fall out, usually.
If you really want to wash them, make sure you dry them properly before use, if not, they won't smell good.
Homemade Black Sesame Powder
50gm black sesame seeds
1 tsp white sesame seeds
1. Place sesame seeds onto a dry pan/wok.
2. Heat the pan on medium low heat and shake/toss the seeds to distribute the heat well.
3. Toast the seeds until the white sesame seeds looks puffy and slightly golden.
4. Let them cool down slightly and mill until fine.
If you want to mill it really fine, you need to mix with dry ingredient like flour or starch. It depends on what you want to do with the sesame powder. But this texture is good enough for cakes, or cookies with some mouthfeel, or for tang yuan.
I sprinkle these over salad and it taste awesome.