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