Grease a 9 × 13-inch baking dish with butter or nonstick cooking spray. For easier removal, line it with parchment paper and leave some overhang.

Place 2 cups of the granulated sugar in a small, heavy-bottomed saucepan.

Cook over low heat, stirring frequently, until the sugar melts completely and turns into a smooth amber caramel. This may take about 20 minutes.

While the caramel is melting, combine the remaining 4 cups of sugar and the heavy cream in a large heavy saucepan or Dutch oven.

Cook the sugar and cream over low heat, stirring occasionally, until the sugar dissolves and the mixture is hot.

Slowly and carefully pour the melted caramelized sugar into the large saucepan while stirring continuously. The mixture may bubble and steam vigorously.

Continue stirring until the caramel and cream mixture becomes smooth and evenly combined.

Increase the heat to medium and attach a candy thermometer to the side of the pot, making sure the tip does not touch the bottom.

Cook, stirring occasionally, until the candy reaches 244°F (118°C), the soft-ball stage.

Remove the pot from the heat and immediately stir in the baking soda. Mix carefully until fully incorporated.

Add the butter and vanilla extract and stir until the butter is completely melted and the mixture is smooth and glossy.

Let the candy mixture rest undisturbed for about 20 minutes.

Add the chopped pecans or walnuts and stir steadily until the mixture begins to thicken and lose its shiny appearance.

Quickly transfer the candy to the prepared baking dish and spread it into an even layer.

Let the candy cool completely for 1–2 hours, or overnight, until firm.

Lift the candy from the pan if using parchment, then cut into about 40 small squares and serve.