First, prepare your baking sheet by lightly greasing it with butter. Keep it ready!
In a large, heavy-bottomed pot, combine the butter, sugar, salt, and vanilla. Heat this mixture over medium-low heat, stirring often until everything is well melted and blended.
Continue to cook, stirring frequently, until the mixture starts to thicken. Use a candy thermometer to monitor the temperature; you're aiming for 260°F.
Once at 260°F, stir in the slivered almonds. Keep cooking and stirring over medium-low heat. The mixture will deepen in color and get thicker, reaching 295°F – this usually takes about 15-18 minutes of consistent stirring.
As soon as it hits 295°F, carefully remove the pot from the heat and quickly stir in the baking soda. Stir vigorously; the mixture will puff up a bit, giving the toffee its delightful texture.
Immediately pour the hot toffee onto your prepared baking sheet. Spread it out evenly and thinly using an offset spatula or the back of a spoon. Be very careful, as the toffee is extremely hot!
While the toffee is still hot, sprinkle the semi-sweet chocolate chips evenly over the top. Let them sit for a few minutes to melt, then gently spread the melted chocolate into an even layer.
Finally, sprinkle the chopped nuts over the melted chocolate.
Let the toffee cool completely at room temperature until it's firm, or speed up the process by placing it in the refrigerator. Once firm, break the slab into bite-sized or larger pieces with your hands. Serve chilled or at room temperature!