Line a rimmed baking sheet with parchment paper or a silicone mat. Set aside.

In a heavy-bottomed saucepan, combine butter, sugar, water, and salt over medium heat.

Stir gently until the butter melts and sugar dissolves. Clip a candy thermometer to the pan, ensuring the bulb is submerged but not touching the bottom.

Bring the mixture to a boil. Do not stir once it reaches a steady boil; gently swirl the pan if needed.

Continue cooking until the mixture reaches 295–300°F (146–149°C), the hard crack stage. This typically takes 8–12 minutes depending on heat and pan.

Remove from heat and quickly stir in the vanilla.

Immediately pour the hot toffee onto the prepared sheet, spreading thinly with a spatula or by tilting the pan. Work swiftly — the toffee sets fast.

Let the toffee sit 1–2 minutes until the surface is glossy but still warm. Sprinkle the chopped chocolate evenly over the top.

After a couple of minutes, when the chocolate softens, spread it evenly with an offset spatula.

While the chocolate is still soft, sprinkle the toasted nuts and any optional garnishes evenly over the top. Lightly press nuts into the chocolate.

Allow toffee to cool completely at room temperature until firm (about 1–2 hours). For faster setting, chill briefly in the refrigerator.

Once set, break into pieces. Store or package as desired.