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In a medium saucepan place butter, cocoa powder, sugar, milk, and vanilla if using, and bring just to a boil over moderate heat. Reduce heat to low, and mix until melted and completely smooth. Remove from heat, add the crushed cookies, and mix well to combine.
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Transfer mixture to a 8x8-inch pan lined with parchment paper, and spread evenly.
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Make the peanut butter ganache: Place milk chocolate and peanut butter in a heat-proof bowl. Microwave in 30-second intervals, mixing in between, until melted and smooth. Spread evenly over the chocolate-cookie mixture.
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Refrigerate for at least 2 hours or until set, then cut into 36 squares. They keep well in the fridge for several days.