Line a baking sheet with foil. Spread out the almonds on the foil - set aside.
Heat the butter, sugar, vanilla and salt in a heavy sauce pan over medium heat until butter is melted.
Increase the temp to med-high & stir constantly until the mixture comes to a boil.
Set the timer for 5 minutes & DO NOT let ANYTHING disturb you while you stir. You must keep stirring your butter/sugar mixture so that it does not burn. (*Note- you must also use a WOODEN spoon.)
After about 2 minutes it should look yellow & after 4 minutes it should look mostly brown & frothy- keep going until it is the color of a brown paper bag & you have cooked it the full 5 minutes.
Immediately pour the mixture onto your foil-lined baking sheet on top of the almonds. (You can lightly scrape the sides of the pan, but do not scrape the gunk on the bottom.)
Move & tilt the baking sheet back & forth to even out the toffee over the almonds.
I like to soak the saucepan with hot water & soap & set aside for easy cleaning later.
Let the almonds/toffee cool. It should harden within a couple of minutes.
Melt the chocolate chips in the microwave in 30-second intervals, stirring inbetween until smooth.
Spread across the toffee with a small spatula.
Sprinkle with slivered almonds if desired.
After it has set up, break into pieces.