1 pound unsalted butter
2 cups granulated sugar
1/2 teaspoon salt
1 teaspoon pure vanilla extract
8 ounces good quality semi-sweet chocolate, chopped
3/4 cup chopped toasted blanched almonds
Line the bottom and sides of a 10-inch x 15-inch baking sheet with parchment paper.
In a large cast iron skillet, melt the butter over medium-high heat. As the butter melts, stir in the sugar and salt. Continue stirring rapidly with a wooden spoon, keeping the sides of pan clean by brushing occasionally with a wet pastry brush. The mixture should bubble as you stir. Cook until the mixture turns a deep auburn brown and registers 300°F on a candy thermometer, taking care not to burn it. Remove from the heat and stir in the vanilla.
Pour the mixture onto the lined baking sheet. Allow it to cool slightly, about 5 minutes, then sprinkle the mixture with the chocolate. When the chocolate looks glossy, spread it with an offset cake spatula, and sprinkle with the nuts. Gently press the nuts into the chocolate.
Cool completely (at least 6 hours) then break the toffee into chunks.