Preparation of Baking Dish: Line a square baking dish (9x9 inches) with parchment paper. Evenly distribute the cup of chopped pecans across the base of the dish. Set aside for later use.
Combining the Ingredients: In a heavy-bottomed pot, combine the granulated sugar, water, kosher salt, and unsalted butter. Place this pot over a medium heat source. Stir the mixture frequently until it reaches a boil.
Monitoring the Toffee Mixture: Continue to heat the mixture, stirring occasionally, until it attains a temperature of 300 degrees Fahrenheit (149 degrees Celsius), also known as the hard crack stage.
Adding Vanilla Extract: Once the desired temperature is reached, remove the pot from the heat. Carefully incorporate the vanilla extract into the mixture.
Formation of the Toffee Layer: Promptly pour the hot toffee mixture over the prepared base of chopped pecans in the baking dish. Utilize a spatula to evenly spread the mixture, ensuring it forms a uniform layer. Allow this to cool slightly for approximately 2 minutes.
Adding the Chocolate Layer: Evenly distribute the milk chocolate chips over the surface of the warm toffee. Cover the dish with foil and wait for about 4 minutes, allowing the chocolate chips to soften. Following this, spread the softened chocolate to form a thin layer over the toffee. Optionally, garnish with finely chopped pecans and a light sprinkling of sea salt.
Cooling and Serving: Place the baking dish in the refrigerator. Allow the toffee to set for a minimum of one hour. Subsequently, remove the toffee from the dish. For a more refined presentation, score the toffee with a knife and then break it into pieces along the scored lines.