Prepare Colored Batter: Make a vanilla cake batter and divide it evenly into four bowls. Color each bowl of batter with red, yellow, green, and blue food coloring respectively.
Layer Batter for Tie-Dye Effect: Scoop one spoonful of each colored batter in rainbow order into each cake pan until about two-thirds full. Shake the pans gently to level the batter, creating a tie-dye pattern in the baked cake.
Bake Cake Layers: Bake the cake layers according to your vanilla cake recipe instructions until done. Let cool completely.
Stack and Crumb Coat: Using Swiss meringue buttercream, stack the leveled cake layers. Apply a thin crumb coat of buttercream around the cake and chill in the refrigerator until firm.
Create Buttercream Transfer Design: Sketch the desired design on an acetate sheet, flip it over, tape it to a cake board, and pipe the Swiss buttercream design onto the acetate. Use a toothpick to drag the buttercream to add dimension.
Apply Transfer to Cake Top: Press the buttercream transfer onto the top of the chilled cake. Refrigerate until the buttercream is firm. Remove the acetate and trim the buttercream to cover just the top of the cake.
Apply Transfer to Cake Sides: Repeat a similar buttercream transfer process for the sides using a wider acetate sheet. Wrap it carefully around the cake, smooth and flatten to ensure contact, then gently peel off the acetate.
Trim Excess Buttercream: Use a knife to carefully trim any excess buttercream from the edges to reveal the clean tie-dye design.