Line a medium baking sheet with parchment paper and set aside.
In a heavy-bottomed saucepan over medium heat, combine butter, sugar, vanilla, and salt.
Let the butter melt and sugar dissolve and then bring to a simmer, then cook, stirring constantly with a whisk, so the butter and sugar don't separate, until mixture turns dark amber, 15 - 20 minutes or 285ºF on a candy thermometer.
Pour toffee mixture into the prepared baking sheet and set aside to harden for 20 minutes.
Gently melt the chocolate and using a spatula spread it thinly and evenly over the hard candy.
Sprinkle the chopped almonds and pecans all over the wet chocolate.
Refrigerate to JUST set the chocolate, about 1 hour.
Then remove from the fridge and break into pieces and enjoy.
Store the toffee at room temperature in an airtight container.
