1. Fill large saucepan 3/4 full with hot water and heat on medium-low heat until bubbles begin to form. Place candy thermometer, pastry brush, and wooden spoon in water to keep utensils clean and reduce heat to low to keep water warm. Fill cup or bowl large enough to hold wooden spoon with ice and cold water. Cut 18-inch length foil and fold lengthwise to 8-inch width. Fit foil into length of 13 x 9-inch baking dish, pushing it into corners and up sides of pan; allow excess to overhang pan edges. Cut 14-inch length foil and fit into width of baking pan in same manner, perpendicular to first sheet. (If using extra-wide foil, fold second sheet lengthwise to 12-inch width.) Fill sink 1-inch-deep with room-temperature water. Place butter and 6 ounces chocolate in freezer.

2. Heat sugars, salt, chocolate syrup, milk, and remaining unfrozen 6 ounces chocolate in large Dutch oven over medium heat, stirring constantly with clean wooden spoon, until chocolate melts, 6 to 12 minutes. Clean wooden spoon with hot running water to remove all sugar crystals; place spoon, handle down, in container filled with ice water. Using pastry brush from hot water bath, wash down small segment of side of pot to remove any sugar crystals. Continue to wash sides of pot, dipping brush back into hot water before wiping down another part of pot, until no sugar crystals remain. Attach candy thermometer to side of pot and continue to cook until syrup boils.

3. Boil sugar syrup until it reaches 236°F, about 30 minutes, then begin testing for doneness. Remove wooden spoon from ice water and dip handle into sugar syrup, sweeping and twisting 2 to 3 times to coat end of handle with syrup. Place spoon handle in ice water, twisting handle so syrup does not slide off, until syrup is cool enough to handle, 5 to 10 seconds. Gather cooled syrup with your fingers and try to roll into ball. If syrup forms soft ball that will flatten when lightly pressed between 2 fingers, remove syrup from heat, about 238°F. (If heat of your fingers heats sugar too quickly, dip them, with syrup, back into water and try to form ball.) If syrup fails to form soft ball, continue to cook, checking every increase of 2°F (between 238 and 242°F) until soft ball is formed, 30 to 40 minutes total cooking time.

4. Remove pot from heat and immediately place in prepared sink. Sprinkle frozen butter, chocolate, and vanilla over top of fudge. Cool pot in sink for 5 minutes. Transfer pot to counter and let cool, without stirring, until fudge reaches 110 to 120°F, about 25 to 35 minutes.

5. Using wooden spoon, stir to fully recombine ingredients. Once recombined, stir mixture 1 or 2 times around perimeter of pot, then lift spoonful of mixture about 1 foot above pot and let it drip back into pot. Repeat stirring and lifting process 4 to 6 times, then let mixture rest 1 minute. Repeat process of stirring, lifting, and resting until mixture begins to lose its shine and becomes difficult to stir, about 8 to 12 minutes total.

6. Quickly transfer fudge to prepared pan and spread in even layer with spatula (alternatively, place piece of wax paper on top of fudge and press into pan using your hands). Cool fudge at room temperature until firm, about 4 hours. Remove fudge from pan using foil and cut into squares.

Makes 4 pounds