Prepare a baking sheet by buttering or lining with parchment paper and set aside. Add the butter, sugar, water, and salt to a medium saucepan (with a heavy bottom).
Heat the pan over medium-low heat, stirring frequently with a wooden spoon, until the butter has melted and the sugar has dissolved. Leave the spoon in the pot and then clip on a candy thermometer at this point if desired (being careful to not let it touch the bottom).
Continue to cook over medium heat until the mixture has reached 285°F and has turned a rich caramel color. Only stir every so often at this point, you don't want to disturb the process too much. When it turns a caramel color immediately remove from heat and stir in the vanilla.
Pour the mixture onto the prepared pan and allow it to spread. Let the toffee cool for 5 minutes, then sprinkle with the chocolate chips and allow to sit for 10 minutes.
Spread the melted chocolate chips with an offset spatula and sprinkle with chopped pecans or almonds.
Let the toffee cool completely, then break into pieces and enjoy.
Notes
Storing and freezing: Store in an airtight container or bag for up to two weeks. I like to layer the toffee between wax paper to prevent sticking. You may also freeze the toffee for about 2 to 3 months.