Preheat oven to 350°F (180°C). Line 2 half sheet baking pans with parchment paper, and grease the sides of the pans.
In a medium bowl, whisk flour, baking powder, baking soda, and salt together. Set aside.
Beat butter and sugar together in a large mixing bowl until lightened in colour and well mixed together, about 3 minutes.
Add eggs and vanilla and mix until well combined, scraping down the sides of the bowl when necessary. Add sour cream and mix until combined.
Add ⅓ of the dry ingredients in and mix until just combined. Add half the milk to the bowl and mix to combine. Continue adding another ⅓ of the dry ingredients, then the rest of the milk, and then the remaining dry ingredients. Mix until just combined after each addition.
Pour cake batter evenly into the prepared baking sheets. Use a spatula to drag cake batter into corners.
Bake for 20-22 minutes, rotating pans halfway through. Cake should be lightly browned at the sides when done, and a toothpick inserted into the center of the cakes should be clean. Let cakes cool in their pans.
Cut out the same large Christmas tree shape out of each cake. Place one tree on a cake board. I have a Christmas tree shape printable in the recipe notes.