Preheat oven to 350 degrees F. Spray half size sheet pan (13" x 18") very well with cooking spray. (Conversely, line sheet pan with parchment paper.) Pour prepared yellow cake mix in pan and spread evenly.
Bake for 15 - 20 minutes, or until cake springs back when pressed lightly. Place cake on rack to cool completely.
Meanwhile, cream together butter and 1/2 cup of the shortening until smooth, using a hand mixer if available. When creamed, add milk and confectioners sugar to bowl and mix well.
Using a cookie cutter or biscuit cutter, cut cooked and cooled cake into desired shape.
Spread filling on bottom of one shape and sandwich on a duplicate. Bottom of each piece of cake should be towards the middle of the cake sandwich. Refrigerate for at least half an hour to set the filling.
Combine white chocolate melting chips and remaining 1/2 cup of vegetable shortening in microwave safe bowl, and heat on high in microwave for one minute. Stir mixture, and continue heating in 20-30 second intervals, stirring after each interval. When smooth, coat cake sandwiches.
Using spatula or large (serving) fork, hold cake over bowl and spoon coating over top and sides. Continue, letting excess drain, until cake is completely coated on top and sides. Place on baking rack or wax paper to set. Continue until all sandwiches are coated.
Melt dark chocolate melting chips and place in small plastic bag. Snip off corner of bag, leaving a tiny hole. Drizzle the chocolate in waves over cakes.
Allow cakes to set completely at room temperature before enjoying.