Assemble work area and measure all ingredients. Spray a parchment-lined baking sheet with non-stick cooking spray. Set aside.
In a medium saucepan, combine sugar, corn syrup, and water. Heat mixture over medium heat, stirring occasionally until the sugar dissolves completely. Liquid should be clear, not cloudy.
1 cup granulated sugar, 1/2 cup light corn syrup, 3 tablespoons water
Attach thermometer to the wall of the saucepan. Bring mixture to a boil and cook, CONSTANTLY MONITORED, until it reaches 290°F (soft crack stage), about 4 to 5 minutes. Note - nothing says fire like an unattended pot of caramel sugar - you absolutely cannot and must not walk away during this process.
Quickly stir in butter, Kosher salt, and seeds. The mixture will initially bubble up. Cook the syrup, stirring, until the thermometer registers 300°F. Sprinkle baking soda over the surface and stir to blend.
1 tablespoon unsalted butter, 1 teaspoon Kosher salt, 1 1/2 cups benne seeds (sesame seeds), 1 teaspoon baking soda
Immediately pour the caramel mixture out onto the prepared baking sheet and smooth out to all corners using a spatula. Sprinkle with coarse sea salt, if desired, and allow to cool COMPLETELY before peeling away parchment and breaking into shards.
1 to 1 1/2 teaspoons coarse sea salt