01 - Preheat oven to 350°F (175°C). Grease and line three 6-inch, two 4-inch, and one 3-inch round cake pans. Bake in batches if necessary.
02 - In a medium bowl, whisk together flour, baking powder, and salt.
03 - In a large bowl, beat softened unsalted butter and granulated sugar until light and airy for about 3 to 4 minutes. Add eggs one at a time, mixing well after each addition. Blend in vanilla extract.
04 - Add the flour mixture to the creamed butter in three additions, alternating with whole milk. Begin and end with the flour mixture. Mix until just combined, avoiding overmixing.
05 - Divide the batter equally into four bowls. Tint each portion with gel food coloring to create vibrant hues. Pour each colored batter into the prepared pans. Use a skewer to gently swirl the batters together for a marbled effect.
06 - Bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean. Allow cakes to cool in the pans for 10 minutes before inverting them onto wire racks to cool completely.
07 - Beat softened butter until smooth. Gradually add 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 necessary. Stack the cooled cake layers from largest to smallest, spreading buttercream between each layer. Offset the placement of each layer to create a whimsical, crooked 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.