Spread almonds out on a baking tray and roast for 8-10 minutes in an oven pre-heated to 350ºF.
Make Toffee
Prepare a medium-large baking tray by lining it with parchment paper. Set aside.
In a medium pot, melt butter with water and corn syrup.
Add sugar and salt and stir briefly to distribute sugar. Do not stir again while mixture caramelizes.
Turn heat to medium high and allow the mixture to cook until it caramelizes. It should look dirty blonde in appearance. If using a digital thermometer - caramelization begins at 154ºC. I used the cold water test to determine readiness - drop about a teaspoon of the mixture in a bowl of cold water and shape with fingers. When the mixture holds the shape and cracks cleanly when broken or bitten, it's ready.
Swirl in the vanilla.
Pour the mixture onto the parchment lined tray in an even later. Allow to cool.
Once completely cooled, used paper towel to blot the surface of the toffee to remove any excess oil.
Use a knife to lightly scratch the surface of the toffee. This helps the chocolate to adhere.
Make Chocolate
Pre-crystallize chocolate (do this while toffee is cooling) and pour half the chocolate onto the scratched surface of the toffee. Use a large offset spatula to evenly spread the chocolate over the toffee. Sprinkle with almonds and allow to set.
Once set, turn the toffee over and repeat the process - wipe, scratch, pour and smooth chocolate, add almonds and allow to set.
Break into pieces and enjoy!
Notes
Toffee can be stored in an airtight container in a cool, dry place for up to two months. Adapted from Chocolate course material.