Step 1: Prepare the pudding. In a large mixing bowl, whisk together the instant vanilla pudding mix and 4 cups of cold milk for about 2 minutes, until the mixture begins to thicken. Cover the bowl and refrigerate for at least 5 minutes to allow it to firm up further.
Step 2: Arrange the first cake layer. Carefully unwrap 6 of the Little Debbie Christmas Tree Cakes. Depending on the size of your trifle dish, you may need to cut some cakes in half or quarters to fit snugly. Arrange them in a single, even layer at the bottom of your clear glass trifle dish.
Step 3: Add the first pudding layer. Spoon half of the prepared vanilla pudding evenly over the layer of Christmas tree cakes. Gently spread the pudding to cover all the cake pieces.
Step 4: Follow with a whipped topping layer. Carefully spread half of the thawed whipped topping over the pudding layer, creating an even, smooth white layer.
Step 5: Repeat the layers. Arrange the remaining 6 Little Debbie Christmas Tree Cakes over the whipped topping, creating your second cake layer. Spoon the remaining vanilla pudding over this second cake layer, spreading it evenly. Finish by spreading the remaining whipped topping over the top of the pudding, creating a beautiful, smooth white canvas.
Step 6: Decorate and chill. Generously sprinkle festive Christmas sprinkles over the top whipped topping layer. Cover the trifle loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow the flavors to meld and the cake to soften.