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1
Preheat the oven to 350.
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2
Line an 8-by-8-inch baking dish with parchment paper, leaving a few inches of overhang all around.
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3
Spread the walnuts on a baking sheet and bake for about 5 minutes, until lightly toasted.
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4
Cool, chop and transfer to a large bowl.
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5
Meanwhile, in a food processor, pulse the biscuits until fine crumbs form; add to the walnuts.
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6
In a resealable plastic bag, crush the pretzels with a rolling pin until small chunks form; add the pretzels to the walnuts.
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7
In a large saucepan, melt the stick of butter over very low heat.
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8
Add the brown sugar and cocoa powder and whisk until smooth.
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9
Slowly drizzle the egg and vanilla into the saucepan, whisking, until thickened, 2 minutes.
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10
Mix in the walnut mixture.
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11
Press the mixture into the prepared baking dish in an even layer and refrigerate the cookie base for 30 minutes.
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12
Spread 1 1/2 pints of the ice cream over the cookie base and freeze for 2 hours.
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13
Reserve the remaining ice cream for another use.
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14
Meanwhile, in a heatproof medium bowl, combine the chocolate, heavy cream, corn syrup and the remaining 2 tablespoons of butter.
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15
Place the bowl over a saucepan of simmering water; make sure the bottom of the bowl does not touch the water.
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16
Stir until the chocolate is melted and the ganache is smooth.
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17
Let cool to room temperature.
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18
Pour the ganache over the ice cream and spread it evenly.
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19
Freeze for 2 hours, until set.
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20
Cut into 8 bars and serve.
