Grease an 8-inch square pan with butter. Line the pan with parchment paper, leaving about 2 inches of overhang to act as a sling. Fill a sink or large bowl with 2 inches of room temperature water.
Combine the sugar, chocolate, 1 1/2 tablespoons of the butter, the half-and-half, and corn syrup in a 4-quart heavy-bottomed saucepan set over medium heat. Stir with a wooden spoon until sugar is dissolved and chocolate is melted. Increase the 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 234ºF.
Remove from the heat and set in the room temperature water. Add the remaining butter, but do not stir. Cool the mixture until it drops to 110ºF, about 10 minutes. Add vanilla and nuts, if desired, and mix until well blended and the shiny texture becomes matte. Pour into the prepared pan. Cool in cool dry place until firm. Cut into 1-inch pieces and store in an airtight container at room temperature for up to 1 week.