Step 1

Lightly coat an 8½x4½" loaf pan with nonstick spray and line with parchment paper, leaving a 2" overhang on long sides. Place a sheet of parchment paper on work surface and lightly coat with nonstick spray. Beat white tahini and ½ tsp. salt in the bowl of a stand mixer fitted with the paddle attachment on low speed just until smooth.

Step 2

Meanwhile, place ⅔ cup sugar in a small saucepan and remaining ⅔ cup sugar in another small saucepan. Add ¼ cup water to each saucepan and set both saucepans over low heat. Cook, stirring with a rubber spatula to dissolve, about 4 minutes. Keep 1 saucepan over low and increase heat to medium-high for other saucepan and fit with thermometer. Cook syrup, brushing down sides of saucepan with a wet pastry brush to dissolve any crystals, until thermometer registers 248°, 7–10 minutes.

Step 3

Immediately remove saucepan with boiling syrup from heat. Increase mixer speed to medium and gradually stream syrup into white tahini, aiming for the space between the sides of the bowl and the paddle. Beat just until halvah comes together in a smooth mass, less than a minute. (Do not overmix or it will be crumbly.) Working quickly, scrape onto prepared parchment and flatten with spatula until ¾" thick. Invert a medium bowl over halvah to keep warm.

Step 4

Rinse any hardened sugar off thermometer; clip to second saucepan. Increase heat to medium-high and cook syrup, brushing down sides of saucepan with wet pastry brush, until thermometer registers 248°, about 4 minutes.

Step 5

While the syrup is cooking, briefly beat black tahini and remaining ½ tsp. salt in the same bowl of stand mixer (no need to clean it out unless you have lots of hardened sugar stuck around sides) until smooth. Stream in syrup and mix just until halvah comes together in a smooth mass.

Step 6

Uncover white halvah (it should still be warm) and scrape black halvah on top; flatten to about the same shape as the white halvah. Using the sides of the parchment to lift edges, fold stacked halvah in half and flatten slightly. Repeat folding and flattening motions 4–5 times, rotating halvah as you work, to create a marbled effect (be careful not to work it too much or mixture will be crumbly). Press into prepared pan. Fold sides of parchment paper over top of halvah and let cool, at least 3 hours.

Step 7

Lift halvah out of pan using parchment overhang. Peel away parchment; cut halvah crosswise into ½"-thick slices.

Step 8

Do Ahead: Halvah can be made 3 days ahead. Leave in pan. Store tightly wrapped at room temperature.