Grease an 8 by 8-inch pan with butter. In a heavy-bottomed saucepan, combine the sugar, chocolate, 1 1/2 tablespoons of the butter, half-and-half, and corn syrup. Over medium heat, stir with a wooden spoon until sugar is dissolved and chocolate is melted. Increase heat and bring to a boil. Reduce heat to medium-low, cover, and boil for 3 minutes. Remove the cover and attach a candy thermometer to the pot. Cook until the thermometer reads 234 degrees F. Remove from the heat and add the remaining butter. Do not stir. Let the mixture cool for 10 minutes or until it drops to 130 degrees F. Add vanilla and nuts, if desired, and mix until well-blended and the shiny texture becomes matte. Pour into the prepared pan. Let sit in cool dry area until firm. Cut into 1-inch pieces and store in an airtight container for up to a week.
Recipe courtesy of Alton Brown