Mix together pudding and milk and whisk for 2 minutes. Refrigerate for 5 minutes until firm. 

Finely crush 15 chocolate sandwich cookies with a food processor and set aside. Coarsely crush, or break apart the remaining 15 chocolate cookies. Set aside. 

To make trees, place wax paper onto a cutting board or baking sheet, about 3" apart. Place green candy melts into a resealable bag but do not seal shut. Microwave for 60 seconds on 100% power and knead the bag until all candy is melted and smooth. Seal the bag shut and cut a small hole into the tip of the bag. Gently drizzle candy over the pretzel sticks to create trees, repeat with a second layer and sprinkle with white nonpareils before moving onto the next tree. 

Repeat until you have the desired number of trees. Add a second "trunk" to the trees, to make them a lot longer so they stick into the trifle better. Do this by adding a small dollop of green candy at the bottom of the tree and attach a second pretzel stick to the tree. Let stand to harden or refrigerate for 10 minutes until hard. 

Slice round cake in half, creating 2 rounds. Place one round into the trifle dish and top with half of the pudding, half of the whipped topping and the crushed chocolate cookies. Repeat layers, ending with a layer of finely crushed chocolate cookies. 

Gently press trees into the trifle as desired and sprinkle with powdered sugar to create snow. 

Serve immediately or refrigerate, loosely covered until ready to serve.