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1
Butter a 9-inch square metal baking pan.
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2
Sift granulated sugar through a sieve into a bowl to remove any lumps or large crystals, then rub brown sugar through sieve into bowl.
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3
Pour sugars into a 2 1/2- to 3-quart heavy saucepan, being careful not to get sugar on side of pan.
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4
Add cream, butter, and 1/4 teaspoon salt and cook over very low heat, stirring frequently with a wooden spoon and washing down any sugar crystals on side of pan with a pastry brush dipped in cold water, until sugar is dissolved (do not let simmer), 10 to 15 minutes.
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5
Clamp on candy thermometer, then boil syrup over moderately high heat, undisturbed, until it registers 236F and a teaspoon of syrup dropped into a small bowl of cold water holds a very soft ball when pressed between your fingers, 3 to 6 minutes.
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6
Remove pan from heat, leaving thermometer in place, and cool, undisturbed, until syrup registers 220F, 1 to 3 minutes.
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7
Stir syrup with cleaned and dried wooden spoon until thickened and creamy, 1 to 2 minutes, then immediately stir in pecans.
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8
Working very fast (syrup hardens quickly), pour into baking pan, scraping sides of saucepan with wooden spoon.
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9
Let mixture harden at room temperature, about 45 minutes.
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10
Cut and break into pieces.
