Step 1: Prep the Pans and Oven
Preheat your oven to 350°F. Line one 8-inch cake pan and five 6-inch pans with parchment, then coat with cooking spray—don’t skimp, or you’ll be prying cake later. This sets the stage for your St. Patrick’s Day Cake.

Step 2: Mix the Dry Ingredients
In a large bowl, whisk the flour, baking powder, and salt until they’re best friends—no clumps allowed. This is the backbone of your cake, so give it a good stir.

Step 3: Cream the Wet Base
In another large bowl, use your handheld mixer on medium-high to beat the granulated sugar and butter until combined—about a minute. Add the oil and keep beating until it’s smooth and creamy, like a dreamy cloud. Add the eggs one at a time, beating well after each, then mix in the vanilla. This mix is the heart of your easy holiday baking adventure.

Step 4: Combine Wet and Dry
Add a third of the dry mix to the wet, beating on medium-low until just a few streaks remain. Pour in the milk and beat until it’s smooth. Add another third of the dry mix, beat again, then add the sour cream and mix well. Finish with the last of the dry ingredients, beating until just combined—don’t overdo it, or the cake gets dense.

Step 5: Color the Batter
Divide the batter evenly into six bowls. Dye one red, one orange, one yellow, one green, one blue, and one purple—use a few drops of food coloring and stir until vibrant. This is where St. Patrick’s Day Cake turns into a festive rainbow dessert.

Step 6: Bake the Layers
Pour the purple batter into the 8-inch pan and bake for 15-18 minutes, until a tester comes out clean. Let it cool. Pour the other colors into the 6-inch pans and bake for 30-35 minutes, testing for doneness. Let them cool too. You’ve got your rainbow stack ready.

Step 7: Make the Frosting
In a large bowl, beat the powdered sugar and butter on medium speed until smooth. Add the heavy cream, vanilla, and salt, beating until fluffy—this buttercream is your canvas.

Step 8: Assemble the Cake
Trim the cake tops if they’re domed. Use a 3-inch cookie cutter to cut centers from the orange, yellow, green, and blue layers. Stack the purple layer on a stand, frost the top lightly, then add the blue, green, yellow, and orange layers, frosting between each. Fill the center with Lucky Charms marshmallows, then top with the red layer. Frost the whole cake thinly for a crumb coat and chill for 30 minutes to 2 hours.

Step 9: Decorate Like a Leprechaun
Dye most of the remaining frosting green and frost the cake smoothly with an offset spatula. Scoop out 1 ½ cups of frosting—dye 1 cup black and ½ cup yellow. Frost a 1-inch black belt near the base (use parchment to keep lines clean), then pipe a yellow buckle with your piping bag. Your St. Patrick’s Day Cake is now a leprechaun hat!