In a medium bowl, whisk together 1¼ cups flour, 1 tsp salt, ½ tsp baking powder and set aside.
Use a double boiler, or if you don't have a double boiler, you can use a metal bowl and a pot. Add about an inch of water to your pot, and find a metal bowl that will sit on rim of the pan without touching the water (you want the steam to heat the bowl).
Add 6oz white chocolate (note: you want melting chocolate here, not chocolate chips), 1 stick butter, ¼ cup tahini to the bowl. Heat over low heat, stirring until the ingredients melt together and mixture is smooth. Remove the bowl from the heat and allow to cool for roughly a minute.
Whisk ½ cup brown sugar into chocolate mixture. It will be grainy and some of the fat may separate a little, but continue to stir until the mixture comes together. Add the egg, 2 egg yolks, and 1 Tbsp vanilla bean extract, mixing until thick, smooth, and glossy.
Add in the flour mixture of dry ingredients and stir until evenly incorporated and thick and glossy. Crumble ½ cup (4oz) of the halvah and gently fold it in, to avoid breaking it up too much. You want a few bigger chunks or crumbles.
Pour into a parchment-paper lined 8"x8" pan and smooth the top. Sprinkle with white sesame seeds and white chocolate chips as desired.
Bake 20-25 minutes as 350°F or until the edges are golden brown. The center should still ever so slightly jiggle. Allow to cool in the pan before removing and slicing.