Line a jelly roll or half-sheet pan with parchment paper and set it aside.
Using a hand mixer, beat together melted butter, peanut butter, granulated sugar, and vanilla extract until smooth and creamy. Beat in powdered sugar.
Press the peanut butter mixture into a sheet pan, spreading to about ½-inch thickness.
Place the sheet pan in the freezer for 15 minutes.
Lift the parchment paper to remove the chilled peanut butter layer from the pan. Place a new sheet of parchment paper on the pan.
Use a cookie cutter to cut out ½” thick Christmas tree shapes from the chilled peanut butter layer.
Place cutouts on a parchment-lined sheet pan. Freeze for 30 minutes.
Scoop up the remnant peanut butter mixture, repress to ½-inch thickness, and cut out additional shapes.
After the cutouts have chilled for 25 minutes, prepare the chocolate coating.
Combine milk chocolate chips and 2 tablespoons of shortening in a heatproof bowl. Heat and stir in 30-second increments in the microwave until the chocolate is smooth and melted completely.
Using a fork, lift one peanut butter cutout at a time and coat it completely with the melted chocolate. Tap excess chocolate off the cutout and place it back onto the cookie sheet.
Work in batches if the peanut butter cutout becomes soft quickly.
Allow chocolate to harden.
For optional drizzle, microwave chocolate chips or chopped pieces and shortening (½ teaspoon per ½ cup) in 30-second increments until melted and smooth.
Scoop melted chocolate into a plastic baggy or piping bag. Snip a tiny corner off the baggy and drizzle in a zigzag motion over chocolate-coated trees. Allow chocolate to set, for about 5 minutes.