Position a rack in the center of the oven and preheat the oven to 350°F.

Place the almonds in a cake or pie pan and toast in the oven for 12 to 14 minutes, until light golden. Stir the almonds every 5 minutes as they are toasting to ensure they brown evenly. Remove the pan from the oven and cool completely on a rack, then chop the almonds finely.

Line a 7 × ll-inch (2-quart) rectangular baking pan with aluminum foil so that the foil fits snuggly in the pan and hangs a bit over the edges. Using a paper towel, oil the foil that lines the inside of the pan. Sprinkle ¾ cup of the chopped toasted almonds over the bottom of the pan.

Melt the butter in a 3-quart heavy-duty saucepan over medium heat. Add the sugar, water, and salt and cook, stirring constantly with a heat-resistant spatula, until the mixture registers 260°F on a candy thermometer. Immediately stir in ½ cup of the chopped almonds and continue cooking the mixture until it becomes golden brown and registers 290° on the candy thermometer. Immediately stir in the vanilla and baking soda. Be careful as the mixture will bubble and foam.

Turn the toffee out into the prepared pan and spread it evenly in the pan and to the edges. Immediately sprinkle the top of the toffee evenly with the chocolate. Let it stand for 2 minutes, until the chocolate begins to melt. Use a small offset spatula to spread the chocolate evenly over the top of the toffee. Sprinkle the top of the chocolate evenly with the remaining ¾ cup of chopped toasted almonds. Let the toffee set up at room temperature until firm, about 30 minutes, then refrigerate for 15 minutes to set the chocolate.

Lift the toffee from the pan by holding the overhanging edges of foil. Peel the foil away from the toffee and break the toffee into small pieces. Serve at room temperature.