Preheat oven to 350°F. Line two baking sheets with parchment paper.
Cream butter and sugar until pale and fluffy. Add avocado oil, then eggs one at a time, mixing well. Stir in vanilla.
In a separate bowl, whisk flour, baking powder, baking soda, and salt.
With the mixer on low, alternate adding dry ingredients with sour cream and buttermilk until just combined.
Divide batter evenly between the two prepared pans. Smooth into thin, even layers and tap pans gently to release air bubbles.
Bake 10–12 minutes, just until set and a toothpick comes out clean. Do not overbake.
Cool completely, then chill briefly in the freezer for cleaner cutting.
Make 7 Minute Frosting
In a heavy-bottom saucepan, combine egg whites, sugar, water, cream of tartar, and salt.
Heat over medium-low, whisking constantly, until mixture reaches 140°F and sugar is fully dissolved.
Transfer immediately to a stand mixer and beat on high speed for 7 minutes, until thick, glossy, and holding soft peaks.
Add vanilla and mix briefly. Transfer frosting to a piping bag and keep upright in a jar while assembling.
Assemble the cakes
Cut cake layers into Christmas tree shapes.
Place one tree cutout on parchment, pipe or spread frosting on top, then gently sandwich with a second tree.
Freeze assembled cakes 30–60 minutes, until firm.
Coat and Decorate
Melt white chocolate with avocado oil in a double boiler, working in two batches (about 12 oz at a time). Add a dash of vanilla if desired.
Dip frozen cakes into melted chocolate, letting excess drip off. Place on parchment.
Stir beet powder into remaining melted chocolate until pink-red. Thin with a little more oil if needed.
Drizzle over cakes and immediately add sprinkles.
Freeze 10 minutes to fully set.
Video
Notes
Use the leftover cake scraps for cake pops: Gather the remaining frosted cake pieces into a large bowl, crumble them with your hands until fully mixed, then roll into small balls. Insert sticks and drizzle with any leftover melted white chocolate. They set beautifully and make the sweetest little bonus treats.