First you’ll want to make your cookie crumbs. If you have a cookie with no creamy middle filling (like a chocolate chip or sugar cookie), then you can put them straight into the food processor. If you do have a filling (like an Oreo or Nutter Butter would), then you’ll want to open the cookie and scrape out the filling with a butter knife first. You want a dry crumb to work with, so the filling has to be removed. Some cookies have a solid middle that you can’t open and get to (like a Milano cookie), but I think that would be OK since the chocolate chip cookies have some chocolate bits mixed in and they turned out fine. Place your cookies into the food processor and blend until it forms a very fine powder. You’ll need 2 cups, so just keep blending cookies until you get there (since all the cookies are different sizes or have two sides per cookie, the number of cookies needed won’t be the same).
In a small saucepan, heat the butter over low heat until melted, and then stir in your sweetened condensed milk and evaporated milk until it’s all melted together. Starting with 1/2 cup of the liquid, pour it into your cookie crumbs and mix together with a spoon. Keep adding small amounts of the liquid until the cookie butter is just wet enough to stay together. Depending on how dry the cookie is to begin with, you’ll need more or less liquid. Really buttery cookies (like the sugar cookies) didn’t need nearly as much as the drier Oreo cookies, so just keep checking and stirring as you add the liquid.
Once your cookie butter is blended, allow it to cool in the fridge for an hour or two. If you want to, you can actually stop at this point, and it will taste delicious. But if you want to make the cookie butter smoother and easier to dip things in, then remove the cookie butter from the fridge and stir very small amounts of water (start with 1/8 teaspoon) into the dough. At first it will look like the water is separating from the dough, but just keep stirring and mashing. You should start to notice that the cookie butter loosens up a bit as you stir in more water. Keep adding and stirring until you get to a consistency you like and refrigerate the mixture again.
Store your cookie butter in the fridge and it should be good for at least a week or two. It may be good for even longer but it usually doesn’t last long enough to find out! And here’s a quick video tutorial of the process: