Preheat oven to 350°F. Line a half baking sheet (18×13) with parchment paper or aluminum foil. Arrange 1 1/2 cups almonds, roughly chopped, in a single even layer on the prepared sheet.

Roast the almonds in the preheated oven for 5 minutes. Remove the pan from the oven and immediately sprinkle the almonds evenly with 1 cup semi-sweet chocolate chips and 1 teaspoon cinnamon. Set the pan aside.

Place a large stock pot (not a small saucepan) on the stove. Add 1 pound salted butter (4 sticks butter), 1/4 cup Karo corn syrup, 1/2 teaspoon salt, 2 1/2 cups sugar, and 1 cup water.

Attach a candy thermometer to the pot so the tip is submerged but not touching the bottom. Bring the mixture to a boil over medium-high heat, then maintain a steady boil. Stir constantly to dissolve the sugar and prevent scorching.

Continue boiling and stirring until the thermometer reaches 300°F (hard-crack stage). This typically takes about 12–16 minutes. Watch the temperature closely as it approaches 275°F because it can rise quickly.

As soon as the toffee reaches 300°F, remove the pot from heat and immediately pour the hot toffee evenly over the prepared almonds, chocolate chips, and cinnamon on the baking sheet. Use a heatproof spatula to spread the toffee into an even layer.

Immediately sprinkle the toffee with the remaining 2 cups semi-sweet chocolate chips. Let the chips sit 1–2 minutes until they soften, then use the spatula to spread the melted chocolate into an even layer over the toffee.

While the chocolate is still soft, sprinkle 1/2 cup pecans, finely chopped, evenly over the top.

Let the toffee cool completely at room temperature until fully hardened. Break into pieces and store in an airtight container at room temperature.