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1
Line the bottom of a greased 13- by 9-inch baking pan with wax paper and grease the paper.
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2
In a heavy saucepan melt the butter with the cocoa, stirring until the mixture is smooth.
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3
Remove the pan from the heat, beat in the brown sugar and the egg, and stir in the flour, the salt, and the vanilla.
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4
Spread the mixture in the baking pan, bake it in the middle of a preheated 350F.
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5
oven for 10 minutes, or until it is just firm to the touch and pulls away slightly from the sides, and let it cool in the pan on a rack.
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6
In a heavy saucepan melt the butter, add the brown sugar and the corn syrup, and bring the mixture to a boil over moderate heat, stirring with a wooden spoon.
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7
Boil the mixture, stirring occasionally, until it reaches the hard-ball stage (260F.
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8
on a candy thermometer), remove the pan from the heat, and add the cream and the walnuts carefully, stirring gently.
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9
Bring the mixture to a boil and boil it until it reaches the soft-ball stage (240F.
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10
on a candy thermometer).
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11
Remove the pan from the heat, let the caramel mixture cool for 1 minute, and stir in the rum.
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12
Pour the topping over the base, spreading it evenly with the wooden spoon, let it cool, and chill the confection, covered with foil, for 2 hours, or until it is firm.
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13
Run a thin knife around the edges of the pan, turn the confection out onto a cutting board, and cut it into bars.
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14
The bars keep, separated by sheets of wax paper, in an airtight container, chilled, for 1 week.
