Step 1

Lightly coat an 8½x4½" loaf pan with nonstick spray and line with parchment paper, leaving a 2" overhang on both of the long sides. Mix tahini, kosher salt, and 2 Tbsp. sesame seeds in a medium bowl to combine; set tahini mixture aside.

Step 2

Cook sugar and ½ cup water in a small saucepan over low heat, stirring with a heatproof rubber spatula, until sugar is dissolved, about 4 minutes. Increase heat to medium-high and fit pan with candy thermometer. Cook syrup, brushing down sides of saucepan with a wet pastry brush as needed to dissolve any crystals that form, until thermometer registers 250°, 7–10 minutes. Immediately remove syrup from heat and gradually stream into reserved tahini, mixing constantly with spatula. Continue to mix just until halva comes together in a smooth mass and starts to pull away from the sides of bowl (less than a minute). Be careful not to overmix or halva will be crumbly. Working quickly, scrape into prepared pan and let cool.

Step 3

Melt chocolate in a heatproof bowl set over a pan of simmering water (do not let bowl touch water), stirring often. Remove from heat. Invert halva onto a wire rack set inside a parchment-lined rimmed baking sheet; peel away and discard parchment. Pour chocolate over halva and sprinkle top with flowers (if using), sea salt, and remaining 2 Tbsp. sesame seeds. Let sit until chocolate is set before serving, about 30 minutes.

Step 4

Do Ahead: Halva can be made 3 days ahead. Store tightly wrapped at room temperature.