Fit a medium-sized pot with your candy thermometer. Line a cookie sheet with parchment paper and set aside.
Add the butter and vanilla to the pot and heat over medium low heat, allowing the butter to almost melt. It's important to watch this and add the sugar at this stage to avoid separation.
12 tablespoons (170g) unsalted butter, 1 teaspoon vanilla extract
Once the butter has almost fully melted, add the brown sugar and salt to the pot. Whisk together continuously and vigorously, do not leave this to sit or it can separate from the butter or worse burn.
1 1/2 cup (300g) brown sugar, 1/2 teaspoon salt
It's important the sugar melts into the butter at this stage. If the sugar doesn't dissolve into the butter, it'll separate and ruin the toffee.
Keep whisking your toffee and watch the temperature- when the toffee reaches 300°F (hard crack stage), remove immediately from the stove. You'll see it go from a dark syrup to getting lighter and marshmallow like.
Immediately pour out the toffee on to the baking sheet lined with parchment, spreading it to about 1/8 inch thickness. You'll have to work fast with this because the toffee hardens fast!
WARNING: Don't ever taste or touch the toffee at this stage, it's incredibly hot and easy to burn yourself.
Allow the toffee to cool for 20 minutes, then chop up with a knife to the size pieces you'd like.
Transfer the toffee bits to a glass jar for storage, and use as needed!
Yields roughly 2 cups of toffee chips