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1
Make chocolate layer: Put oven rack in middle position and preheat oven to 350F Line a 9-inch square baking pan with foil, allowing 2 inches of foil to hang over 2 opposite ends of pan, and lightly brush foil (except overhang) with oil.
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2
Melt chocolate with butter, sugar, and salt in a double boiler or a large metal bowl set over a pan of barely simmering water, whisking occasionally, until smooth.
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3
Remove top of double boiler or bowl from heat and whisk in eggs and vanilla, then stir in peanuts.
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4
Pour batter into baking pan and bake until set and edges are slightly puffed, about 15 minutes.
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5
Transfer cake in pan to a rack to cool completely.
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6
Make peanut butter layer: Beat cream cheese, peanut butter, confectioners sugar, and salt in a bowl with an electric mixer at medium speed until combined well, about 4 minutes.
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7
Beat cream with vanilla in a bowl with cleaned beaters at medium-high speed until it just holds stiff peaks, then gently fold into peanut butter mixture until just combined.
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8
Spread mixture evenly on top of chocolate layer and freeze, pan covered with foil, until firm, about 2 hours.
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9
Make hot fudge sauce: Bring cream, syrup, sugar, cocoa, salt, and chocolate to a boil in a 1-quart heavy saucepan over moderate heat, stirring until chocolate is melted.
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10
Reduce heat and simmer, stirring frequently, until thickened, about 3 minutes, then remove from heat.
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11
Stir in vanilla until incorporated.
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12
Cool sauce to warm.
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13
To serve: Carefully lift cake from pan by grasping overhang.
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14
Trim 1/4 inch from edges of cake and discard, then cut into 8 bars.
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15
Peel off foil and serve bars with sauce.
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16
Cooks' notes:.
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17
Hot fudge sauce can be made 1 week ahead and cooled completely, then chilled in an airtight container or jar.
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18
Reheat before using.
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19
Leftover bars can be frozen, individually wrapped in plastic wrap, up to 4 days.
