Line a baking sheet with parchment paper or a silicon baking mat, such as a Silpat. If using nuts, sprinkle 1/2 cup roasted pecans on top and spread evenly.
Using a heavy-bottomed saucepan over medium heat, melt brown sugar and butter, stirring constantly. Boil until a candy thermometer reaches 300º F. Immediately remove from heat and pour over roasted nuts. Top with chocolate chips and allow chips to begin melting. Spread chocolate evenly over top of toffee with a silicon spatula. Allow to cool for about five minutes before adding remaining roasted pecans.
If omitting the roasted pecans from your recipe, consider sprinkling top of chocolate with sea salt.
Allow to cool completely to harden - overnight on counter or about 20 minutes in the freezer. Lift hardened toffee off parchment paper or silpat and break into candy pieces.
Optional: for thicker toffee pieces (as pictured), I line a 7x11 casserole dish with parchment paper rather than using a baking sheet. Freezing time increases to 30 minutes.