Place sugars, corn syrup, half-and-half and chocolate in large heavy saucepan. Bring to boil, stirring to melt chocolate and dissolve sugar. Reduce heat to moderate and continue cooking, stirring occasionally, until syrup reaches 248F on candy thermometer (firm-ball stage). Remove from heat. Quickly stir in butter and vanilla just until blended and butter melts. Pour into well-greased 8x8x2" pan. Cool; cut in small squares. If desired, top each square with pecan half or almond slivers. Wrap individually in plastic wrap or foil. Store in cool, dry place. Makes about 1-1/2 pounds.