Adapted from At My Table, by Nigella Lawson. These meringue-based cookies are gluten-free, dairy-free, and Passover-friendly. Oh, and they're delicious.
Line one large baking sheet or two medium baking sheets with parchment paper. Preheat the oven to 400 F.
Spread the sugar on one of the baking sheets in an even layer, and heat in the preheated oven for 7-8 minutes. Remove the pan from the oven and reduce the oven temperature to 350 F.
Meanwhile, using a hand mixer or stand mixer with whisk attachment, whip the egg whites with a pinch of salt until soft peaks form. Turn the speed to low and gradually add the heated sugar. Turn the mixer to high and continue beating until the mixture is glossy and forms firm peaks.
Use a rubber or silicone spatula to fold in the potato starch, vinegar, and vanilla bean paste. Now fold in the crumbled halva and other "mix-ins" -- mini chocolate chips, sesame praline candy, etc. Finally, swirl in the 2 tablespoons of tahini -- don't try to fully incorporate it.
Use a spoon to drop 12-14 mounds of the meringue mixture on the lined baking sheet(s). If you want, swirl a bit more tahini into the top of each mound, and sprinkle over some sesame seeds.
Place the cookies in the oven, and then place a sticky note on or near the oven door to remind you that the cookies are in there! Now, turn off the oven, and forget about the cookies for the next 5 hours, or overnight. The cookies will be cooked by the residual heat, yielding crisp outsides and marshmallow-y insides.