Heat oven to 350°. Combine and mix together pretzels and melted butter in a large bowl. Press mixture evenly onto the bottom of a 13x9-inch baking pan or pyrex dish. I found it is better to refrigerate the "crust" for a good hour or so (sometimes overnight) before spreading the mixture on top of it so it does not move around while still warm.
Pour sweetened condensed milk and chopped unsweetened chocolate into a small sauce pan over low heat. Constantly stir the mixture until the chocolate is completely melted and mixed in with the milk. Pour the mixture over the pretzel "crust" and spread evenly over the pretzel layer.
Top the chocolate layer with the marshmallows, then the coconut, and then the nuts. With a piece of wax paper, press firmly down with your hands on the chocolate layer.
Bake 25-30 minutes or until the top is lightly browned. Cool completely in the pan.
In a small saucepan, melt the shortening. Add the chocolate and peanut butter chips to the shortening and stir constantly until melted. Drizzle over the top of the mixture and refrigerate at least 15 minutes or until set. Cut into bars and serve.