Prepare the baking sheet by lining it with parchment paper or lightly greasing it.

In a medium saucepan over medium heat, combine the granulated sugar, butter, water, and salt. Stir continuously until it comes to a boil, then allow to boil without stirring until it reaches a deep amber color, about 300°F (150°C).

Remove from heat and stir in the vanilla extract, then fold in half of the chopped nuts.

Carefully pour the hot toffee mixture onto the prepared baking sheet and spread it evenly.

While still hot, sprinkle chocolate chips over the top and allow them to soften before spreading evenly.

Garnish with remaining chopped nuts and optional flaked sea salt. Let cool completely at room temperature before breaking into pieces.