Combine unsalted butter, white sugar, and salt in a large, heavy-bottomed saucepan over medium heat. Stir constantly in one direction until the butter is melted, about 5 minutes.

Meanwhile, line a 9x13-inch baking dish with a sheet of parchment paper, allowing it to overhang two sides of the dish.

Once the butter has melted, stop stirring and bring the mixture to a boil. Cook, stirring only 2 to 3 times, until it turns a dark amber color and the temperature reaches 285°F (137°C) on a candy thermometer, which takes about 20 to 30 minutes.

Immediately pour the toffee into the prepared baking dish. Sprinkle semisweet chocolate chips on top and let sit until they soften, about 1 to 2 minutes. Use a spatula to spread the softened chocolate into a thin, even layer. Sprinkle chopped almonds over the chocolate and press down slightly.

Place the toffee in the refrigerator until set, approximately 1 hour.

Lift the hardened toffee out of the baking dish using the parchment paper. Break it into pieces and store in an airtight container.