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Prepare a 9" square pan by coating the bottom and sides with a little butter or non-stick spray.
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Melt chocolate in saucepan over low heat.
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Raise heat to medium and add sugar, milk, corn syrup and salt and stir until it boils.
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As soon as it begins to boil, stop stirring and attach a candy thermometer to the side of the pot making sure it doesn't make contact with the bottom.
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While fudge is cooking, use a pastry brush dipped in water to wash down the sides of the pan to dissolve and remove any sugar crystals that form
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Cook without stirring until temperature reaches 235 degrees ("soft ball" stage). If temperature increase slows or stops rising, you need to increase the heat. You should have a consistent boil.
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Remove from heat and let sit, undisturbed, until temperature falls to 110 degrees
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Add butter and vanilla and begin to stir with wooden spoon
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Stir continuously until surface changes from shiny to dull and fudge begins to "set up" or thicken. This may take 15 to 20 minutes.
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Pour into prepared pan and press in with buttered spatula or your fingers.
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After fudge is completely cooled, cut into 16 squares.