Line a baking sheet with parchment paper and set aside.
In a heavy saucepan over medium heat, melt butter and sugar together, stirring constantly.
Continue cooking and stirring until the mixture turns deep golden and reaches hard-crack stage.
Remove from heat and stir in vanilla and salt.
Immediately pour the toffee onto the prepared baking sheet and spread evenly.
Sprinkle chocolate chips over the hot toffee and let sit until melted, then spread evenly.
Top with nuts and flaky salt if using.
Let cool completely, then break into pieces