Line a large rimmed baking sheet with parchment paper and set aside. Measure and prepare all ingredients before you begin cooking, this is a very sensitive and hands-on recipe and you'll want everything ready to go because you will need to work quickly.
Toast the nuts separately but either baking at 350° on a parchment-lined baking sheet for 5 to 7 minutes OR in a skillet over medium heat for about 5 minutes.
In a large heavy-bottomed stainless or enameled saucepan over medium heat, combine butter, sugar, and water and cook, stirring constantly with a rubber spatula, until butter and sugar are melted and mixture begins to boil, about 5 minutes.
Reduce the heat to medium-low and continu°e to cook, stirring constantly and scraping the bottom of the pot, until mixture is golden brown in color and registers at least 305°F but not more than 310°F, 10 to 15 minutes.
Working quickly, remove the saucepan from heat and stir in 1 cup of roughly chopped almonds.
Transfer toffee to a rimmed baking sheet lined with parchment paper and spread into a thin, even layer about ⅛ to ¼-inch thick. Let the toffee sit for 2 minutes.
Sprinkle chocolate chips over the hot toffee and let sit until softened, about 2 minutes. Use a rubber spatula to spread the chocolate all over the surface of the toffee.
Sprinkle toffee evenly with the pecan pieces and salt (if using) and let sit for 10 minutes.
Transfer to refrigerator and chill until chocolate is hardened, about 1 hour.
Break toffee into roughly 2-inch pieces, transfer to an airtight container, and refrigerate for up to one month.