Position oven racks in the top and bottom thirds of the oven, and preheat to 350 degrees F. Line 2 rimmed baking sheets with parchment paper.
Whisk together the flour, baking powder, baking soda and salt in a small bowl.
Beat the butter and sugar in the bowl of a stand mixer on medium-high speed until fluffy, about 2 minutes. Reduce the speed to medium, beat in the egg and vanilla until well combined, then beat in the tahini until smooth. Reduce the speed to low, then beat in the flour mixture until the dough comes together (it will be quite soft). Cover and refrigerate for 30 minutes (the dough will be soft but rollable).
Put the sesame seeds in a small bowl. Roll teaspoonfuls of dough into 1-inch balls, roll them in the sesame seeds and space them about 1 1/2 inches apart on the prepared baking sheets.
Bake the cookies until just puffed, lightly golden brown and just firm, about 12 minutes, rotating the pans halfway through. Let the cookies cool in the pan on a wire rack for 5 minutes, then transfer them to the rack to cool completely.
To assemble: Spread 1/2 teaspoon jam on a cookie, then top with another cookie, pressing gently. Keep in an airtight container for 2 days; they will soften overnight.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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