Line a square, 8-inch cake pan with parchment paper.
Put the chocolate chips, condensed milk and butter into large bowl set over just simmering water, and melt slowly. Don’t let the bowl touch the water. Once melted, take off the heat and add the powdered sugar. Break in 4 of the cookies, and bring it all together with a wooden spoon. Transfer the mixture to the prepared pan and smooth over, then break up the remaining cookie and press that into the top of the fudge. Chill for 2 hours then cut into chunks.