Let’s break down this complex process into manageable steps:

Cake:

1. Preheat and Prep Pans: Preheat oven to 350°F (175°C). Line 1 (8″) cake pan and 5 (6″) cake pans with parchment paper; coat the pans with cooking spray.

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

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

4. Add Oil: Add the vegetable oil and beat until smooth and creamy.

5. Add Eggs: Add the eggs, one at a time, beating well after each addition, then beat in the vanilla extract.

6. Alternate Dry and Wet 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. Be careful not to overmix.

7. Divide and Color Batter: Divide the batter evenly among 6 bowls. Dye one bowl of batter red, one orange, one yellow, one green, one blue, and one purple, using food coloring. Gel food coloring is recommended for its vibrancy.

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

9. Bake Remaining Layers: Pour the remaining colored batters into the prepared 6″ cake pans. Bake until a tester inserted into the center comes out clean, 30 to 35 minutes. Let cool completely.

10. Make Ahead (Optional): Cake layers can be baked 1 day ahead. Wrap in plastic and refrigerate, or freeze up to 2 weeks.

Frosting & Assembly:

1. Make Buttercream: 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.

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

3. Cut Out Centers: Using a 3″ round cookie cutter, cut out the center of the orange, yellow, green, and blue cake layers. This creates the cavity for the Lucky Charms marshmallows.

4. Assemble 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.

5. Fill with Marshmallows: Pour the Lucky Charms marshmallows into the center of the cake (the cavity created by the cookie cutter).

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

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

8. Chill: Refrigerate the cake until well chilled, at least 30 minutes or up to 2 hours. This helps to set the crumb coat and makes the cake easier to frost.

9. Color Remaining Frosting: Scoop out about 1 ½ cups of the buttercream. Dye the remaining buttercream green.

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

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

12. Create Black Belt: Using an offset spatula, frost a 1″-thick belt with the black frosting at the base of the hat, above where the brim would be. To help create a smooth line, you can wrap the cake in parchment paper above where you’ll frost, covering the area where the brim of the hat would be with parchment as well.

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

14. Serve or Chill: Serve immediately, or chill until ready to serve.