You can use an 8x8-inch baking pan. Line it with parchment paper, and spraying with a nonstick cooking spray.
Cut the butter into cubes, and add it to a large, heavy-bottomed saucepan over a low setting. Heat, stirring occasionally, until it is almost melted.
Add the sugar and whipping cream, and salt. stirring together. Raise the setting to medium, and continue to heat, stirring, until the sugar has melted.
Continue to cook, stirring more frequently, until it just comes to a boil. Simmer until a candy thermometer, inserted into the fudge, reads 235 F.
Remove the saucepan from the burner, and add the white chocolate chips and lemon zest, stirring briskly until the chocolate is melted.
Add a few drops of yellow food coloring as desired for a lemony look.
Transfer the fudge into the pan you’ve prepared, and smooth over the top to even it out.
Allow the fudge to set at room temperature for 3 or 4 hours, or in the fridge for 1 to 2 hours.
Cut into squares and serve.