Preheat oven to 350°F. Grease or line rimmed sheet cake pans with parchment paper.

In a stand mixer, cream the softened butter and sugar for 3–5 minutes until light and fluffy, scraping down the sides as needed.

In a separate bowl, whisk together the eggs, egg whites, sour cream, milk, and vanilla.

In another bowl, whisk together the gluten-free flour, baking powder, baking soda, potato starch, guar gum, and salt.

With the mixer on low, alternate adding the wet and dry ingredients to the butter mixture, beginning and ending with the wet ingredients. Mix just until combined.

Divide the batter between the prepared pans and use an angled spatula to spread into an even layer.

Bake for 20–25 minutes, or until lightly golden and a toothpick inserted in the center comes out with a few moist crumbs.

Cool in the pans for 30 minutes. Use Christmas tree cookie cutters to cut out an even number of trees, then carefully transfer to a cooling rack.

Allow the cakes to cool completely before assembling.

In a stand mixer fitted with the paddle attachment, beat the softened butter for 2 minutes until creamy. Add the marshmallow fluff and mix until combined.

Gradually add the powdered sugar, 1 cup at a time, mixing on low and scraping down the sides as needed.

Add the heavy cream, vanilla, and salt. Mix until incorporated.

Increase the speed to medium-high and beat for approximately 5 minutes until light and fluffy.

Adjust consistency with additional cream if too thick or powdered sugar if too soft.

Transfer the frosting to a piping bag fitted with your preferred tip.

Pipe an even layer of buttercream onto the flat side of one Christmas tree cake. Place a second tree on top, gently pressing the layers together.

Repeat with the remaining cakes until all trees are assembled.

Place the trees on a cooling rack over a parchment-lined baking sheet. Freeze for approximately 45 minutes, or until the frosting is firm.

For the White Chocolate Coating

Combine the white chocolate and coconut oil in a microwave-safe bowl. Microwave in 30-second increments, stirring between each, until completely melted and smooth. Alternatively, melt together using a double boiler.

Pour the melted chocolate into a shallow bowl for easier dipping.

Remove the Christmas trees from the freezer and let sit for approximately 10 minutes before dipping.

Carefully dip each tree into the melted white chocolate, making sure the top and sides are completely coated. Use two forks or a dipping tool to lift the cakes out, allowing excess chocolate to drip off.

Immediately add your favorite holiday sprinkles while the chocolate is still wet.

Transfer to parchment paper and allow the chocolate to harden. Refrigerate for 10–15 minutes to speed up setting.

Store in an airtight container in the refrigerator. Let sit at room temperature for 15–20 minutes before serving.