Preheat your oven to 350°F (175°C). Grease and flour your small tree-shaped pans or line a rectangular pan if making squares.

Cream the butter and 1¾ cup sugar until pale and light, about 3 minutes. The mixture should feel airy and hold soft peaks.

Add the vegetable oil, then the whole eggs one at a time, beating briefly after each. Mix in the egg whites until the batter looks glossy.

Stir in the sour cream and 1 tablespoon vanilla. Mix until smooth.

Whisk together cake flour, baking powder, and 1 teaspoon salt. Add to the wet mix in three parts, alternating with the buttermilk. Start and finish with flour.

Pour batter into prepared pans, filling each about two-thirds full. Tap the pans to remove large air pockets.

Bake 18 to 22 minutes for small tree pans, or 28 to 32 minutes for a larger sheet, until tops are pale gold and a toothpick comes out with moist crumbs.

Cool the cakes in pans for 10 minutes, then turn out onto a rack. Allow to cool completely before filling.

In a medium bowl, soften 1/2 cup butter with a mixer until smooth. Add 1 cup marshmallow fluff and beat until light.

Add 3 cups powdered sugar gradually, beating on low at first, then medium until smooth and glossy.

Stir in 1 teaspoon vanilla, 2 tablespoons milk or cream, and 1/4 teaspoon salt. Chill the filling briefly if it seems too soft.

If you baked a sheet, cut the cakes into tree shapes or small rectangles. For tree pans, sandwich two halves around a spoonful of filling.

Place assembled cakes on a parchment-lined tray and chill for 20 to 30 minutes to firm the filling.

Melt white chocolate with coconut oil in 30-second bursts, stirring until smooth.

Dip each chilled cake into the melted white chocolate or pour the chocolate over the cakes for a more even coat.

Set the coated cakes on a rack or parchment. Chill briefly to set the chocolate, about 15 minutes.