1. Line an 8-inch square pan with parchment paper, leaving enough overhang to lift the fudge out easily later.

2. Add the sugar, cocoa powder, whole milk, butter, and salt to a heavy saucepan over medium heat. Stir until the butter melts and the mixture looks smooth.

3. Bring the mixture to a full boil, stirring as needed so it cooks evenly without scorching.

4. Let it boil for 4 to 5 minutes, until the mixture looks glossy and slightly thickened.

5. Remove the pan from the heat and stir in the vanilla extract and chocolate chips until smooth.

6. Fold in the chopped walnuts or pecans if using.

7. Let the mixture cool briefly, then beat it with a wooden spoon until it starts to thicken and lose a little shine.

8. Pour the fudge into the prepared pan and smooth the top.

9. Let it cool completely, then chill until firm.

10. Lift it out of the pan and cut into 16 squares.