Line a muffin tin with 9 to 12 muffin liners; set aside. In a medium bowl, add the dark chocolate chips. Place in the microwave for 1 minute; remove and stir. Microwave for an additional 30 seconds and then stir vigorously until completely smooth.
In another medium bowl, add the white chocolate chips. Place in the microwave for 1 minute; remove and stir. If needed, microwave for an additional 30 seconds and stir until completely smooth. (With white chocolate I usually only need 1 minute.)
In a small bowl, add the tahini, brown rice syrup or honey and a pinch of salt. Barely stir them together (Striations of the syrup or honey are a-ok!)
Add a scant tablespoon of dark chocolate to half the liners. Add a scant tablespoon of white chocolate to the remaining liners. Smack the muffin tin on the counter to even out the chocolate.
Using a fork, add little drops of white chocolate to the dark chocolate cups and little drops of dark chocolate to the white chocolate cups. Using a knife, run it through the drops, creating swirls. Move quickly because the chocolate will set (depending on how cold your house is).
Transfer to the freezer to chill for 5 minutes. Remove it from the freezer and add a teaspoon of the tahini mixture to each cup. Transfer back to the freezer to set for an additional 5 minutes.
I found that the marbled tops worked best to do them in small batches of 2 to 3 cups. So, top 3 of the white chocolate cups with a scant tablespoon of white chocolate. Add a few drops of dark chocolate to the surface of the white chocolate cups and run your knife through them, creating swirls. Repeat this process, working in batches of 2 or 3 cups, until you’ve created swirls in all of the cups. Give it a gentle tap on the counter to even out the chocolate and transfer to the freezer for 10 minutes to set. Keep in the fridge until you’re ready to serve.
If you don't own a microwave, be sure to invest in really good white chocolate so it doesn’t seize on you and you also may want to consider adding a teaspoon of heavy milk to the white chocolate (but this is only if you’re doing it via stovetop).