Line a 9-by-13-inch baking pan with parchment paper or foil and lightly grease it. Have the white chocolate and chopped nuts ready before cooking the sugar mixture.

Place the butter, sugar, water, and salt in a heavy saucepan over medium heat. Stir until the butter melts and the sugar dissolves.

Continue cooking, stirring frequently, until the mixture reaches 300–310°F (149–154°C), the hard-crack stage. The toffee should be deep golden brown, which usually takes 10–15 minutes after boiling begins.

Remove the saucepan from the heat and carefully stir in the vanilla. Expect the hot mixture to bubble vigorously.

Immediately pour the toffee into the prepared pan and spread it into an even layer. Work promptly because the candy begins setting quickly.

Sprinkle the white chocolate over the hot toffee and let it stand for 2–3 minutes. Spread the softened chocolate evenly across the surface.

Sprinkle the chopped nuts over the chocolate and gently press them in. Add the flaky sea salt, if desired.

Cool the candy at room temperature or refrigerate it until completely firm, about 1 hour. Break or cut it into pieces and store it in an airtight container.