Step 1

Line an 8x8-inch square baking pan with parchment paper so that it hangs at least 1 inch over two opposite sides.

Step 2

Place 1 1/2 cups granulated sugar and 1/2 cup water in a small saucepan and bring to a boil over medium heat. Continue to boil without stirring until 250°F, 6 to 10 minutes more. If sugar crystals form on the sides of the pan, brush them down with a damp pastry brush.

Step 3

Meanwhile, place 1 1/2 cups room temperature tahini, 1 teaspoon vanilla extract, and 1/2 teaspoon kosher salt in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until smooth and combined, about 1 minute. Turn off the mixer until the sugar syrup is ready.

Step 4

With the mixer on the lowest speed, carefully pour the syrup into the tahini mixture and beat just until it comes together into a smooth and shiny paste, 30 seconds to 1 minute from when the syrup first goes in. Err on the side of under mixing rather than over mixing, which will result in an overly dry, crumbly texture. Take the bowl off the mixer and give the mixture one or two quick folds with a flexible spatula to make sure everything is well combined if needed.

Step 5

Working quickly, transfer the halva mixture to the baking pan, push it to the edges of the pan, and smooth the top with the spatula. Sprinkle with 1 teaspoon black sesame seeds if using. Let cool to room temperature, about 2 hours.

Step 6

When ready to serve, grasp the excess parchment and lift the halva slab onto a cutting board. Cut into 25 squares or desired shapes. Wrap leftovers tightly in plastic wrap and refrigerate for up to 3 months. Do not freeze.