Preheat the oven to 350°F. Line a jelly roll pan with parchment paper.

In a large mixing bowl, combine yellow cake mix, eggs, egg yolk, milk, and melted butter. Mix until smooth.

Spread the batter evenly into the prepared pan and bake for 12–15 minutes, or until a toothpick inserted in the center comes out clean. Allow cake to cool completely.

In a medium bowl, beat together the marshmallow fluff, softened butter, and powdered sugar until well combined. Add milk and whip until light and fluffy.

Lift the cooled cake from the pan using the parchment paper. Cut the cake in half (short direction).

Spread the filling across one half of the cake. Place the other half on top, creating a cake sandwich.

Use a Christmas tree–shaped cookie cutter to cut out tree cakes. Place them on a parchment-lined baking sheet and freeze for 30 minutes.

Melt white almond bark in the microwave in 30-second intervals at half power until smooth.

Dip each chilled tree into the melted almond bark, tapping off excess. Place on parchment and sprinkle with green sanding sugar.

Melt red candy melts and transfer to a piping or zip-top bag. Snip the corner and drizzle over the trees to create stripes.

Allow coating to fully harden, then serve and enjoy!