4 oz (about 1/2 cup) granulated sugar
4 oz (about 1/2 cup, packed) dark brown sugar
1 oz (about 4 teaspoons) light corn syrup
1/2 teaspoon kosher salt (I used Mortons. If you use Diamond, you may need a touch more)
4 oz (1/2 cup) heavy cream
Butter an 8" square metal pan with straight sides. Line with nonstick foil, pressing it down to smooth it out against the pan as much as you can. Put the pan on a cooling rack convenient to the stove. Cut the butter into small pieces and put in a heavy-bottomed pot over medium heat. Once the butter has melted, stir in the two sugars, corn syrup, and kosher salt. Stir well until mixture comes to a boil. Alternately stir with either a wooden spoon or a high heat spatula and swirl the pan to keep the mixture moving. Either clip a candy thermometer onto the side of your pan or keep checking the temperature with an instant read thermometer. When the candy reaches 250F, add the heavy cream. The mixture will sputter some, so add it carefully and then stir it in well. Continue cooking the candy, stirring and swirling almost constantly until it reaches 295-297F. Carefully pour the candy into the prepared pan. You can use your spatula to gently help the last bit of the candy pour out, but don't scrape to clean the pot out. Allow the candy to cool for about 10 minutes. (If you use a smaller pan, it will take a few minutes longer. You want the candy to be pliable but still be able to hold its shape. Use the foil to ease the candy out of the pan and put it on a cutting board, foil and all. Peel the foil back from the sides of the candy and, using a pizza wheel, score the candy into 64 pieces in an 8 x 8 grid. You can run the wheel over your lines a few times. Try to make the score marks fairly deep--about halfway through the candy. A little butter will leak out in the score marks, but worry not. It will reabsorb once the candy has cooled completely and cured. Move the scored slab of candy back onto the cooling rack. Let the candy cool for about an hour or so until completely firm. Allow the candy to sit in a dry spot where nobody will bother it for several hours or overnight. Then break the candy into pieces along the score lines. It will be easy to break. Store candy in an airtight container at room temperature for up to a week or so.