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1
In a 3-to4-quart heavy-bottomed pan over medium-low heat, stir sugar, cream, and corn syrup, continually scraping the bottom of the pan with a heatproof flexible spatula, until sugar is completely dissolved, about 15 minutes.
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2
Stir in unsweetened and bittersweet chocolate until melted.
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3
Increase heat to medium and bring mixture to a simmer.
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4
Cook, occasionally stirring mixture and brushing down sides of pan with a wet pastry brush, until mixture reaches 235 degrees on a candy thermometer.
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5
Remove from heat and pour into the bowl of standing mixer or large bowl.
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6
Add butter and vanilla but DO NOT STIR; insert candy thermometer and let mixture stand undisturbed until cooled to 110 degrees, 1 1/2 to 2 hours.
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7
Line 9-inch pan with foil; (Release foil works great) Lightly butter foil.
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8
With the paddle attachment of standing mixer or a sturdy wooden spoon beat the chocolate vigorously (on highspeed if using mixer; reduce speed if motor starts to labor) until mixture thickens and loses its glossy sheen, about 5 minutes with a mixer, about 10 minutes by hand.
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9
Scrap into pan, smooth top, and chill until firm to the touch,at least 2 hours,or up to 1 day.
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10
Lift foil to remove fudge from pan; cut fudge into 1 inch squares.
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11
Store cut fudge airtight in the refrigerator up to 1 week.
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12
To keep longer (up to 1 month) wrap uncut fudge airtight and chill; cut into squares as you want to use it.
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13
Serve at room temperature.
