Step 1

Arrange a rack in the middle of the oven and heat the oven to 350°F.

Step 2

Arrange 1 cup whole almonds or pecans in a single layer on a rimmed baking sheet. Bake until fragrant and lightly toasted, about 5 minutes. Transfer to a cutting board and finely chop when cool.

Step 3

Use wrappers from 1 pound unsalted butter to grease the edges and rim of the same rimmed baking sheet and the sides of a medium heavy-bottomed saucepan. Line the bottom of the baking sheet with parchment paper or a silicone baking mat and place on a heatproof surface.

Step 4

Place the butter, 2 1/2 cups granulated sugar, and 1/2 teaspoon kosher salt in the saucepan over medium heat. When the butter is almost melted completely, stir gently with a wooden spoon, being careful not to splash the edges with the sugar.

Step 5

Attach a candy or deep-fry thermometer to the pan and cook until the mixture darkens to a light caramel color or the color of peanut butter, and reaches 295°F to 300°F, 17 to 21 minutes. Stir the mixture occasionally to break up any hot spots and to emulsify the butter and sugar. After every stir, rinse the wooden spoon well to remove all sugar crystals so the candy mixture doesn’t seed. If you notice sugar crystals around the edge of the pan above the syrup, brush those areas with a damp pastry brush.

Step 6

Immediately pour the mixture onto the prepared baking sheet. Let cool until the bubbles subside and the toffee begins to set, 3 to 4 minutes. Sprinkle evenly with 2 cups semi-sweet chocolate chips (careful, the baking sheet will still be very hot). Let until softened and melted, about 5 minutes. Use an offset metal spatula or a silicone spatula to spread the chocolate into an even layer. Sprinkle with the nuts, pressing gently into the chocolate.

Step 7

Let cool completely at room temperature, at least 3 hours. If the chocolate does not harden at room temperature, refrigerate for 20 minutes once the toffee is completely cool. Use your hands or a chef’s knife to break the toffee into large pieces.