Line a 9x13-inch baking dish with parchment paper, leaving an overhang on two sides to assist with removal once the toffee has set.

In a large, heavy-bottomed saucepan, combine the unsalted butter, granulated sugar, and salt. Place over medium heat and stir continuously in one direction until the butter is fully melted. This process should take approximately 5 minutes.

Once the butter has melted, cease stirring and allow the mixture to reach a boil. Continue cooking without stirring, except for 2 to 3 gentle stirs to prevent sticking. Watch carefully as the mixture changes color to a deep amber and reaches 285°F (137°C) on a candy thermometer. This step typically requires 20 to 30 minutes.

Immediately and carefully pour the hot toffee mixture into the prepared baking dish. Spread evenly as needed.

Sprinkle the semisweet chocolate chips evenly over the hot toffee. Allow the heat to soften the chocolate for 1 to 2 minutes. Once softened, use a spatula to spread the chocolate into a smooth, even layer.

Evenly sprinkle the finely chopped almonds over the melted chocolate. Press them gently into the surface to ensure they adhere properly.

Transfer the baking dish to a refrigerator and allow the toffee to set completely. This typically takes about 1 hour.

Once fully hardened, lift the toffee out of the baking dish using the parchment paper. Break the toffee into smaller, irregular pieces by hand.

Place the pieces in an airtight container, layering them between sheets of parchment paper to prevent sticking. Store at room temperature or in the refrigerator.