Put the oreo cookies in a food processor and pulse with the blade attachment until they are ground to a fine crumb. If you don't have a food processor, make your crushed oreos by putting your oreos in a ziploc bag, and rolling with a rolling pin until they're all finely crushed. Set aside.
Measure 4 cups of rice krispies into a mixing bowl and set aside.
Prepare a surface for the finished cookies by spreading a piece of parchment paper or wax paper on a table, and having two large spoons and a glass of cold water ready.
Place butter and marshmallows in a large saucepan. Cook over a medium heat, stirring occasionally until all the marshmallows are melted and combined. Turn off the heat.
Add enough black food colouring to make the marshmallow mixture black. Add in the oreo crumbs, and rice krispies and stir until completely combined and black. If needed add a little more food colouring.
Using two spoons dipped in cold water to help prevent the cookies from sticking, take a spoonful of the hot cookie mixture, and use the second spoon to help remove it onto the parchment paper and create misshapen balls of the cookie mixture.
Let the cookies set and cool slightly, but while still warm, use your fingers to compress the cookies slightly and make them look like lumps of coal.
Let cool completely and store in an airtight container.