Preheat oven to 350ºF. Line two 12x16-inch baking pans with parchment paper and spray with nonstick cooking spray.

In a mixing bowl, whisk together the flour, baking powder, baking soda, and salt until combined; set aside.

In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium speed until lightened and creamy. Beat in the eggs, one at a time, until combined, scraping down the sides and bottom of the bowl as needed. Beat in the sour cream and vanilla on medium speed.

Pour the dry ingredients into the wet ingredients and mix until combined. Add the buttermilk and mix until just combined, scraping down the sides and bottom of the bowl as needed.

Divide batter evenly among the two pans, and spread out evenly with a knife or offset spatula. Bake for 15-17 minutes. Allow to cool in the pan for cool completely.

Combine the hot water and salt in a small bowl and stir until dissolved. Let mixture cool.

In the bowl of a stand mixer, fitted with a paddle attachment, or with a hand mixer in a large bowl, beat together the marshmallow creme, shortening, and butter until combined. Add in the powdered sugar and vanilla and mix on high speed until fluffy--about 2 minutes. Add the salt solution to the mixture and mix until combined.

Flip one of the cooled cakes onto a large cutting board and remove the parchment paper. Repeat with the other cake. Flip both cakes back over so that the smooth side is facing up once again. Spread the filling evenly over one of the cakes. Top with the second cake.** Place cake sandwich in freezer for at least 1 hour.

Once chilled, use a thick christmas tree cookie stencil to punch out individual cakes. If you don't have a cookie stencil, cut into squares or triangles with a sharp knife. Once cut, freeze cakes for another 30 minutes.

In large microwave-safe bowl, heat the white chocolate and coconut oil together until smooth and melted. Using a fork to lift the individual cakes, gently dip the bottom of the cakes into the melted chocolate (not all the way under), then use a spoon to ladle the chocolate over the top and sides. Place onto wax paper-lined baking sheet. Immediately top with green sugar sprinkles. Repeat with the rest of the cakes.

Melt candy melts according to package instructions, and transfer to a small ziploc bag. Cut off the tip/corner and drizzle on top of each cake. Refrigerate for about 5 minutes to allow chocolate to set up. Serve and enjoy!