Line a 9x13-inch baking pan with parchment paper, letting it hang over the edges.

Attach a candy thermometer to the side of a heavy-bottomed saucepan.

Combine unsalted butter, granulated sugar, light brown sugar, and kosher salt in the saucepan and heat over medium-low, stirring until melted (3-5 minutes).

Once boiling, do not stir and let boil for 20-25 minutes until it reaches 290-300°F and turns deep amber.

Remove from heat, stir in vanilla, and pour into the pan.

Sprinkle chocolate chips over warm toffee and cover loosely with foil. Let sit for 5 minutes.

Spread melted chocolate evenly across the toffee. If desired, sprinkle with walnuts, sea salt, or sprinkles while soft.

Let sit until cool and hard, about 1-2 hours, then cut into pieces.