Combine 1 cup unsalted butter chunks and 8 ounces roughly chopped dark chocolate in a medium, heatproof bowl. Set the bowl containing the ingredients over a small saucepot filled with a little bit of water (ensuring that the bottom of the bowl does not touch the water) to make a double boiler. Place over medium heat and stir the ingredients occasionally until both the butter and chocolate have melted completely. Remove bowl from heat and set aside on a wire rack to cool to room temperature.
When the butter and chocolate have cooled, center a rack in the oven and preheat to 350 (F). Prepare a 9 x 13-inch pan by lining the bottom and sides with parchment paper. Set aside.
In the bowl of a freestanding electric mixer fitted with a paddle attachment, combine 1 1/2 cups granulated sugar and 4 large eggs. Beat on medium-high speed until pale and creamy, at least 5 minutes. Reduce the mixer to its lowest setting and slowly pour in the butter and chocolate mixture (from the 1st step). Once all the mixture has been added, stop the mixer and sprinkle 1 scant cup all-purpose flour, 1/3 cup unsweetened cocoa powder, and 1 teaspoon kosher salt over the mixture. Use a rubber spatula to fold the dry ingredients into the chocolate mixture by hand, being careful not to overmix — fold just until the dry ingredients have been incorporated into the mixture. At this point, it's okay to still have 1 or 2 small flour streaks left. Sprinkle 1 cup walnuts over the mixture and fold just until they are incorporated throughout the mixture. Carefully pour the mixture into the prepared baking pan and use an offset spatula to spread it evenly across the pan and smoothen the top. Use a spoon to scoop 1/2 cup tahini paste on top of the batter in about 12 places, before using the back of the spoon to swirl it a just a little into the mix. Don't swirl it in too much tough! You want it to be a little uneven for texture. Dot the chunks of the halva on the brownie surface, pushing each chunk a little into the batter so that is about 50% submerged but still visible. Sprinkle 1/3 cup halva floss across the top to finish.
Transfer to the preheated oven and bake for about 30 to 35 minutes, until the top is crisp but the middle still has a slight wobble and is gooey inside. The brownies will seem a bit undercooked, but they will firm up as they cool down. Serve warm and gooey, or at room temperature. Enjoy!