Gather your ingredients: chopped pecans or almonds, butter, granulated sugar, water, salt, vanilla, and chocolate chips. Prepare a 9x9 inch baking dish lined with parchment paper.

Line your baking dish with parchment paper and spread 1 cup of chopped pecans or almonds on the bottom.

In a pot, combine butter, granulated sugar, water, and salt. Heat on medium, stirring frequently until the butter melts and the mixture bubbles gently.

Attach a candy thermometer and allow the mixture to heat up, stirring occasionally, until it reaches 300°F (149°C).

Remove the pot from heat once it reaches 300°F to prevent overcooking. Be cautious as the mixture is extremely hot.

Stir in 1 teaspoon of vanilla extract into the hot toffee mixture until evenly mixed.

Pour the hot toffee mixture over the layer of pecans or almonds in the dish and spread it evenly.

After 2 minutes of cooling, sprinkle 1 cup of chocolate chips over the warm toffee.

Cover the dish with foil or a baking sheet and let it sit for 4 minutes to melt the chocolate chips.

Uncover and swirl the melted chocolate with a knife or spatula to create a marbled appearance.

Sprinkle finely chopped pecans and optional flaked sea salt on top of the chocolate.

Allow the toffee to cool completely for about 4 hours at room temperature.

Once set, lift the toffee out of the pan using the parchment paper edges and cut into squares.