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1
Coat the back of a baking sheet with 2 tablespoons of the butter; set aside.
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2
Combine the sugar, the water, molasses, and corn syrup in a 3-quart heavy-bottomed saucepan over high heat, and cook without stirring until the mixture registers 268F on a sugar thermometer (about 10 minutes).
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3
Brush down the sides of the pan two times with a pastry brush dipped in warm water to prevent the sugar from crystallizing.
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4
Add 1 tablespoon of the butter, the salt, and walnuts, and cook the mixture until it registers 300F on a sugar thermometer, stirring constantly with a wooden spoon or heatproof spatula.
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5
Remove the saucepan from the heat and with a long-handled wooden spoon rapidly stir in the baking soda and vanilla.
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6
Be careful; the mixture will bubble and foam.
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7
Immediately turn the mixture onto the buttered baking sheet and spread out thin with the wooden spoon.
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8
Let the brittle cool for 5 minutes, then loosen it from the pan with a flexible-blade spatula.
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9
Butter your fingertips with the remaining 1 tablespoon butter and stretch the brittle as thin as possible.
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10
Let it cool completely (about 30 minutes), then break it into pieces with your hands.
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11
In a tightly covered container, the brittle will keep for 1 week at room temperature.
