Prepare a small baking sheet or container lined with wax paper.
Using a hand mixer or a stand mixer fitted with a paddle attachment on medium-high speed, mix together the unsalted butter, peanut butter, and marshmallow fluff until completely combined. Scrape down the sides of the bowl as needed.
Switch the mixer to low speed and gradually add in the confectioners sugar. Mix together until the peanut butter filling forms (it should be soft, creamy, and a little sticky).
Take a few spoonfuls and fill each Christmas tree cavity with the peanut butter mixture. Fill them almost to the top.
Freeze the peanut butter trees until they are firm enough to dip into the chocolate - about 1 hour, but overnight is my preference.
Using a double boiler (fill the bottom boiler with about 1″-1.5″ water), melt the wax over low-heat uncovered until melted. Then add in the bag of chocolate chips and stir continuously until melted.
Using 2 forks, carefully dip each tree into the melted chocolate then set back onto the wax paper. Repeat until all the trees are done.
After the chocolate has hardened, trim off any excess chocolate from the bottom (optional step).
Store the chocolate covered peanut butter trees in the fridge or you can freeze them (in a container) for up to a month.