Prep. Line a 9x13-inch pan with parchment paper, leaving some overhang for easy lifting.
Combine toffee ingredients. In a large heavy saucepan, combine sugar, butter, water, corn syrup, and salt. Heat over medium, stirring often, until the butter melts. Attach a candy thermometer without touching the bottom.
Cook to hard crack. Continue cooking and stirring until the mixture reaches 295–300°F (hard crack stage), about 10–15 minutes.
Add vanilla and pour. Remove from heat and stir in vanilla. Quickly pour the toffee into the prepared pan and spread evenly. Let sit at room temperature until firm, about 1–2 hours.
Add topping layer. Melt the chocolate and spread a thin layer over the cooled toffee. Sprinkle with chopped almonds. Allow chocolate to set at room temperature or refrigerate for 30 minutes. Lift parchment to remove, then cut into pieces.
Cool and break. Allow chocolate to set at room temperature or refrigerate for 30 minutes. Lift parchment to remove, then cut into pieces.
Notes
A candy thermometer makes this easy, but you can also use the cold water method: drop a little hot mixture into cold water. If it hardens into a brittle ball, it's ready. But a candy thermometer is so much better!