Let’s break down the process into easy-to-follow steps. This is a multi-step recipe, so we’ll divide it into sections for clarity.

Make the Cake:

Step 1: Preheat and Prepare Pans. Preheat your oven to 350°F (175°C). Line one 8-inch cake pan and five 6-inch cake pans with parchment paper. Coat the pans with cooking spray.

Step 2: Whisk Dry Ingredients. In a large bowl, whisk together the flour, baking powder, and salt.

Step 3: Cream Butter and Sugar. In another large bowl, using a handheld mixer on medium-high speed (or in the bowl of a stand mixer), beat the granulated sugar and softened butter until combined.

Step 4: Add Oil, Eggs, and Vanilla. Add the oil and beat until smooth and creamy. Add the eggs one at a time, beating well after each addition. Then, beat in the vanilla extract.

Step 5: Combine Wet and Dry Ingredients. Add about one-third of the dry ingredients to the wet ingredients and beat on medium-low speed until just a few dry streaks remain. Add the milk and beat until incorporated. Add another one-third of the dry ingredients and beat on medium-low speed until just a few dry streaks remain. Add the sour cream and beat until incorporated. Add the remaining dry ingredients and beat until just combined.

Step 6: Divide and Color Batter. Divide the batter evenly among six bowls. Dye one bowl of batter red, one orange, one yellow, one green, one blue, and one purple.

Step 7: Bake Purple Layer. Pour the purple batter into the prepared 8-inch cake pan. Bake until a tester inserted into the center comes out clean, 15 to 18 minutes. Let cool completely.

Step 8: Bake Remaining Layers. Pour the remaining colored batters into the prepared 6-inch cake pans. Bake these cakes until a tester inserted into the center comes out clean, 30 to 35 minutes. Let cool completely.

Step 9: Make Ahead (Optional). The cake layers can be baked 1 day ahead. Wrap them in plastic wrap and refrigerate, or freeze for up to 2 weeks.

Make the Frosting & Assembly:

Step 1: Make Buttercream Frosting. In a large bowl, using a handheld mixer on medium speed, beat the powdered sugar and softened butter until smooth. Add the heavy cream, vanilla extract, and salt, and beat until combined.

Step 2: Trim Cake Layers. Trim the tops of the cooled cake layers as needed to make them level.

Step 3: Cut Out Centers. Using a 3-inch round cookie cutter, cut out the center of the orange, yellow, green, and blue cake layers.

Step 4: Assemble the Cake. Place the purple layer on a cake stand or platter. Frost the top with a thin layer of buttercream. Top with the blue cake layer. Frost with more buttercream and repeat with the green, yellow, and orange cake layers in that order, frosting the top of the orange layer with buttercream.

Step 5: Add Marshmallows. Pour the Lucky Charm marshmallows into the center of the cake (the hole created by cutting out the centers of the colored layers).

Step 6: Top with Red Layer. Top with the red cake layer.

Step 7: Crumb Coat. Frost the top and outside of the cake with a thin layer of buttercream to create a crumb coat. This will seal in any loose crumbs and create a smooth surface for the final frosting.

Step 8: Chill. Refrigerate the cake until well chilled, at least 30 minutes or up to 2 hours.

Step 9: Divide and Color Remaining Frosting. Scoop out about 1 ½ cups of the buttercream and set it aside. Dye the remaining buttercream green.

Step 10: Frost with Green Buttercream. Using an offset spatula, frost the cake with the green buttercream, smoothing it with a bench scraper.

Step 11: Divide and Color Reserved Buttercream. Divide the reserved buttercream into about 1 cup and ½ cup. Dye the 1 cup portion black and the ½ cup portion yellow.

Step 12: Create Black Belt. Using an offset spatula, frost a 1-inch thick belt with the black frosting at the base of the “hat” (above the “rim” created by the larger purple layer). To help create a smooth line, you can wrap the cake in parchment paper above where you’ll frost, covering the “rim” of the hat with parchment as well.

Step 13: Add Yellow Buckle. Transfer the yellow buttercream to a piping bag fitted with a medium round tip. Pipe a buckle onto the black belt.

These steps are detailed, but not too difficult to do.