Prepare the Pan: Line a 9x13-inch baking pan with parchment paper for easy release.
Mix Ingredients: Combine the unsalted butter, granulated sugar, light brown sugar, and kosher salt in a heavy saucepan. Heat over medium-low until it comes to a boil.
Achieve Hard Crack Stage: Once boiling, stop stirring! Allow to boil undisturbed for 20-25 minutes until it reaches the hard crack stage (300°F). Add vanilla extract just before it's done.
Pour and Rest: Carefully pour the hot toffee into your prepared pan and let it rest for about 5 minutes to firm up.
Add Chocolate: Sprinkle chocolate chips over the warm toffee, seal with foil, and let it sit for 5 minutes to melt.
Spread and Top: Spread the melted chocolate across the toffee and add nuts, sea salt, or sprinkles if desired.
Chill and Cut: Transfer to the fridge and chill for about 30 minutes. Once set, cut into bite-sized pieces and enjoy!
