Fill the bottom portion of a double boiler (see note) with water. Make sure when you place the top portion on, the water doesn't touch the bottom. Bring the water to a boil over medium-high heat. Reduce heat to medium-low so that the water is simmering.
Place the top portion of the double boiler over the simmering water. Add the milk/dark chocolate, white chocolate, and butterscotch melting wafers and baking chips together and melt, stirring with a rubber spatula.
Use small tongs to dip the sandwiches in the melted chocolate mixture. Shake off excess chocolate by tapping the tongs on the edge of the double boiler. Place the cookie on a baking sheet. You can also drop the cookie in the chocolate, flip it with a fork, and then use the fork to scoop up the cookie.
Once the cookie sheet is full, place it in the freezer until the cookies are frozen. Store in a zip lock bag in the freezer.
*I've gotten the best results with Ghirardelli melting wafers and Guittard's butterscotch chips for these cookies.
A double boiler is a piece of kitchen equipement that's often used with delicate ingredients where direct heat could cause them to seize or separate (like chocolate). It consists of two pots. The first, or bottom, is larger and looks like a regular saucepan. The second, or top, is more shallow and nests inside the larger one.
But if you don't have one, don't worry. Just set a metal mixing bowl over a regular saucepan. Whether you use a double boiler or a saucepan and mixing bowl, make sure the bottom of the top bowl doesn't touch the surface of the water.