01 - Preheat your oven to 350°F (175°C). Prepare three 6-inch, two 4-inch, and one 3-inch round cake pans by greasing and lining them. Bake in batches if necessary.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt.
03 - In a large bowl, beat the softened unsalted butter and granulated sugar until the mixture is light and airy, approximately 3 to 4 minutes. Gradually add the large eggs one at a time, ensuring thorough incorporation after each addition. Blend in the vanilla extract.
04 - Add the prepared flour mixture to the creamed butter in three separate additions, alternating with the whole milk. Begin and conclude with the flour mixture. Mix until just combined, being careful not to overmix the batter.
05 - Divide the cake batter equally into four separate bowls. Tint each portion with gel food coloring to achieve vibrant hues. Pour each colored batter into the prepared cake pans. Use a skewer to gently swirl the batters together, creating a marbled effect.
06 - Bake the cakes for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in their pans for 10 minutes before inverting them onto wire racks to cool completely.
07 - Beat the softened butter until smooth. Gradually incorporate the sifted powdered sugar, whole milk, vanilla extract, and salt, beating until the mixture is light and fluffy. Divide the buttercream into portions and tint each with gel food coloring.
08 - Level each cake layer if required. Stack the cooled cake layers from largest to smallest, spreading a layer of buttercream between each tier. Offset the placement of each layer to create a whimsical, crooked Christmas tree silhouette.
09 - Using a star or round piping tip, pipe decorative buttercream branches in swirling patterns around the stacked cake layers, alternating the vibrant colors. Adorn the finished tree with edible glitter, assorted candies, and Christmas sprinkles.
10 - If desired, insert a pretzel stick at the base of the cake to serve as the tree trunk.