Add the powdered sugar to the food processor and blend everything for about 30 seconds. The mixture will look sandy and gray. Next, add the walnut oil, vanilla, and salt. Blend everything again for another 3 minutes or so, scraping down the sides of the bowl about every 30 seconds. The black sesame paste should be similar to the consistency of natural peanut butter that’s been recently stirred–not too runny, and not too thick. If the paste is still very thick, feel free to add another tablespoon of oil and blend. Taste the paste and adjust the flavors to your liking, adding more sugar or salt, if necessary.