Prep the some of the garnish by blending one of the chocolate sandwich cookies for the garnish by crushing one until it becomes powdered. I added mine to the blender with the 8 chocolate sandwich cookies for the milkshake. Place the crushed chocolate sandwich cookie in a bowl. Set aside. 

Add frosting to the rim of the milkshake glasses and add whole chocolate cookie sandwiches to the frosting. Fill in the gaps with the crushed chocolate sandwich cookie. Place the glasses in the freezer. 

Blend the 8 chocolate sandwich cookies for the milkshake. Add the remaining ingredients for the milkshake and blend until smooth. 

Take the chocolate sauce and drizzle on the inside walls of the glasses. Fill with the milkshake. 

Top with the whipped cream and garnish with remaining crushed chocolate sandwich cookie and another whole chocolate sandwich cookie. Enjoy immediately.