Line a rimmed half-sheet pan with parchment paper.
Melt butter in a wide pot over medium-high heat. Stir in sugar and salt as the butter melts.
Stir constantly with a silicone spatula, regularly scraping down the sides. It will bubble and hiss as you stir.
Cook until the toffee mixture has achieved a deep, golden brown color. It will darken a bit more once you take it off the heat, so stop a shade lighter than you want the end product to be.
Once you’ve reached your desired level of doneness, remove pot from the heat and stir in the vanilla. It will sputter and spit when you add the vanilla, so be careful.
Pour the toffee into your prepared baking sheet. Let cool for approximately 10 minutes before evenly sprinkling the chocolate across the top. When the chocolate begins to melt, use an offset spatula to spread it out into an even layer.
Sprinkle nuts across the warm chocolate and pat gently with your palms to embed the nuts into the chocolate.
Let the toffee cool for 4-6 hours, until it becomes stiff and brittle. Break into pieces. Store in an airtight container. This candy will keep well at room temperature for weeks (if it lasts that long).