Melt the butter in an 8 quart saucepan with the sugar, salt and water over gentle heat. Low heat is important to prevent separation later.
Stir the mixture constantly while heating over medium-high heat. The butter and sugar will bubble and foam as the water boils off. This can take several minutes because butter contains a decent amount of water. The volume of the mixture will increase dramatically at this point and the temperature should be relatively constant at a few degrees above the boiling point of water. Once the water has boiled off, the mixture will collapse and thicken. The temperature will start to rise again. The goal is to remove the pan from the heat once the mixture passes 300°F and before it reaches 320°F. Use an instant read thermometer or candy thermometer to keep track of the temperature as you heat and stir as the temperature can change rapidly once the water boils off. When the mixture reaches 300°F, remove it from the heat and stir in the vanilla extract. Pour the mixture onto either a silicone baking mat or a large sheet of wax paper set on top of a sheet pan. The silicone baking mat is probably easiest to work with since it wont slide around on the sheet pan. If youre using wax paper, one way to keep it in place is to dab the underside of the four corners with a little bit of butter. That will help the paper stay put while the toffee is poured on. Right after pouring, use a spatula (again silicone works best) to spread the toffee into a rough rectangular shape. While the toffee is still hot, sprinkle the surface with the chocolate chips. Wait until the bottoms of the chips start to turn shiny and dark brown as they melt from the heat of the toffee, about two minutes. Use your spatula to spread the chocolate. If the chocolate is still mostly solid, wait another minute before attempting to spread again. Spread the chocolate so that it covers the toffee. Sprinkle the chocolate surface with chopped almonds. Lightly press down on those pieces that are barely touching the surface of the chocolate. Let the toffee cool for about twenty minutes until the sheet pan returns to room temperature. Slip the pan into the refrigerator to cool down and set for at least thirty minutes.
Remove from the refrigerator and peel the toffee from the baking mat or parchment paper. Working quickly so the chocolate doesnt melt too much, break the toffee into chunks of the desired size and place into an airtight container.
Appeared in print on 12.24.08.