Heat the oven to 350F. Line two quarter sheet pans with parchment and grease lightly with nonstick spray (or use one half sheet pan).

Whisk the flour, baking powder and salt in a small bowl.

Beat the 1/2 cup butter and the sugar with an electric mixer for 3 to 5 minutes, until light and fluffy. Beat in the eggs one at a time, then the vanilla.

Add the dry ingredients and the buttermilk in turns, starting and ending with the dry ingredients. Mix only until combined, finishing with a spatula.

Spread the batter evenly in the pans. Bake 10 to 12 minutes, until the cake no longer jiggles and is just golden at the edges. Cool 5 minutes in the pans, then turn out onto parchment and cool completely. If you used one half sheet, cut it in half the short way.

Beat the 1 cup butter with the marshmallow fluff until no lumps of butter remain. Slowly beat in the 3 cups confectioners' sugar, then the vanilla extract.

Set one cake layer on a parchment-lined sheet, spread the filling over it and top with the second layer. Freeze at least 1 hour.

Cut out trees with the cookie cutter (about 8). Freeze them on the lined sheet at least 2 more hours so they do not fall apart in the coating.

Stir the 1/2 cup confectioners' sugar, water and red food dye until smooth and spoon it into a piping bag.

Melt the candy melts with the coconut oil in 30-second bursts, stirring between each, until smooth.

Dip one tree at a time with two forks, let the extra drip off and set it on parchment. After 30 to 45 seconds, before the coating sets, scatter on green sprinkles. Pipe red stripes across each tree.