Preheat the oven to 325°F (165°C). Line an 8 x 8-inch (20 x 20cm) square baking pan with parchment paper.
In the bowl of a food processor, pulse together the flour, sugar, and salt just to combine. Add the butter, tahini, and vanilla. Pulse together until a crumbly dough forms.
Press the dough into the prepared pan and smooth the top with a spatula. Use a fork to poke holes the dough. Bake until the top is light golden brown, 25 to 35 minutes.
Scatter the chocolate pieces on top of the shortbread. Turn off the heat and return the pan to the oven for 1 to 2 minutes to soften the chocolate. Using an offset spatula, spread the softened chocolate into an even layer over the shortbread. Sprinkle with the crumbled halvah and allow to cool to room temperature in the pan before cutting.
