Line a large baking sheet (approx. 13x18 inches) with parchment paper. Lightly grease the parchment paper with butter or non-stick spray. Set aside.

In a heavy-bottomed 3-quart saucepan, combine the unsalted butter pieces, granulated sugar, water, light corn syrup, and fine sea salt. Place over medium heat.

Stir continuously with a rubber spatula until the butter is completely melted and the sugar is fully dissolved. This should take about 3-5 minutes. Ensure no sugar crystals remain on the sides of the pan.

Attach a candy thermometer to the side of the saucepan, ensuring the tip does not touch the bottom. Increase the heat to medium-high and bring the mixture to a boil. Allow it to boil without stirring, occasionally swirling the pan gently if needed, until the temperature reaches 300-310°F (149-154°C), which is the hard crack stage. This process typically takes 10-15 minutes.

Once the temperature reaches 300-310°F, immediately remove the saucepan from the heat. Carefully stir in the vanilla extract. The mixture will bubble vigorously, so be cautious.

Quickly and carefully pour the hot toffee mixture onto the prepared baking sheet. Using an offset spatula or the back of a spoon, spread the toffee into a thin, even layer, about 1/8 to 1/4 inch thick. Work quickly as it will begin to set.

Let the toffee cool completely at room temperature, undisturbed, for at least 1 hour, or until it is very hard and brittle. Do not refrigerate, as this can cause it to become sticky.

Once completely cooled and hardened, carefully peel the toffee off the parchment paper. Place it in a large, sturdy zip-top bag. Using a rolling pin or a meat mallet, gently break the toffee into small, irregular bits. Avoid smashing too hard to prevent very fine dust.

If desired, melt the semisweet chocolate chips in a microwave-safe bowl in 30-second intervals, stirring until smooth. Spread the melted chocolate evenly over the cooled toffee slab before breaking it, or drizzle over the bits. If spreading, sprinkle with chopped nuts while the chocolate is still wet. Let the chocolate set completely before breaking or serving.