Line a square or rectangular baking dish (8 x 8 for thicker bars, 9 x 13 for thinner ones) with aluminum foil or parchment paper, leaving an overhang.
Smash your chocolate sandwich cookies so that you have crushed, bite-sized chunks. (The finer you mash the cookies, the more likely they bars will be gray -- though they'll also be easier to cut with smaller cookie pieces!) Use a food processor if you're feeling civilized, or have at it with a rolling pin. (For neatness, you can double bag the cookies before crushing them.)
Melt the white chocolate gently on the stovetop, or place it in a microwave-safe bowl and heat it in short increments, stirring after each, until melted and smooth.
Mix the crushed cookies into the melted white chocolate and pour into the prepared pan. Place into the freezer to set, at least 2 hours. Take the pan out of the freezer, remove the bars using the overhanging foil or parchment, and allow to sit and warm up for 5 to 10 minutes before slicing with a sharp knife (dip the knife in hot water if you're having difficulty cutting).
Store the bars in the refrigerator or the freezer.