Preheat your oven to 350°F. Spray an 18 x 13-inch metal cookie pan with cooking spray, line with parchment paper, and spray again. Set aside.
In a medium size bowl, add the cake flour, salt, and baking powder. set aside
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, oil, and sugar on medium speed until fluffy and smooth, about 3 minutes. Scrape the bottom and sides of the bowl with a rubber spatula.
Start adding the egg whites and whole eggs, one at a time, beating on medium speed after each addition for about 30 seconds. Scrape the bowl after each addition. Once all the eggs are added, beat for 30 more seconds.
Add the sour cream and vanilla extract. Beat on medium-low speed for 1 minute. If the mixture looks clumpy, don't worry; it will smooth out once the flour mixture is added.
On low speed, add ⅓ of the flour mixture and ½ of the buttermilk to the butter mixture, mixing until almost incorporated. Add another ⅓ of the flour and the remaining buttermilk, mixing just until a few streaks of flour remain. Add the final ⅓ of the flour mixture and mix just until combined.
Scrape the bottom and sides of the bowl to ensure everything is evenly mixed.
Pour the batter into the prepared pan and bake for 35-40 minutes, or until a toothpick comes out with just a few moist crumbs and the top is golden brown. Let the cake cool completely in the pan.
For the Marshmallow Filling
In a large mixing bowl, beat the softened butter with an electric mixer on medium speed until smooth and creamy, about 2 minutes.
Add the marshmallow fluff to the bowl and mix until fully combined, scraping down the sides as needed.
Gradually add the powdered sugar, 1 cup at a time, mixing on low speed until incorporated. After all the powdered sugar is added, increase the mixer speed to medium-high and beat until light and fluffy, about 2-3 minutes.
Add the vanilla extract, heavy cream, and salt, mixing until just combined. Scoop out about ¼ cup of the filling, tint with red food coloring, and place in a piping bag. Place the remaining filling in a separate piping bag with a small hole cut. Set aside.
Assembly
Once the cake has cooled, place it in the fridge or freezer to firm up for easier cutting while you get the chocolate melted.
In a large, heatproof bowl, microwave the white chocolate or almond bark in 20-second intervals, stirring between each interval, until fully melted. Stir in the coconut oil until smooth and fully incorporated. Tip: Keep the white chocolate mixture in a warm spot while dipping, or place it over a warm water bath to maintain a smooth consistency.
Use a tree-shaped cookie cutter to cut the chilled cake into 20 tree shapes (or as many as your cutter allows).
Carefully slice each tree horizontally with a serrated knife.
Pipe the marshmallow filling onto one half of each tree, piping all the way to the edges. Place the second half on top, gently pressing to secure.
When ready to dip, take a tree and dip it into the melted white chocolate, using a fork to lift it out and place it on parchment paper. Let the chocolate harden before piping the red frosting.
Using the red frosting, pipe a zigzag design at an angle over each tree and add green sprinkles. Serve and enjoy!