Lucky Marshmallow Sheet Cake
February 12, 2026
A soft green cake topped with fluffy marshmallow frosting and a shamrock-rainbow sprinkle combo!
Lucky Marshmallow Sheet Cake is all about color, frosting, and a little bit of magic. The green cake stays soft and moist, the marshmallow frosting is light and fluffy, and the Lucky Charms marshmallows on top bring the whole thing together. It’s cheerful, festive, and really hard to resist.
Flavor Profile
The cake is light, vanilla-forward, and super soft thanks to the sour cream. The marshmallow frosting is fluffy, sweet, and creamy without being heavy. Every bite feels like a classic snack cake with a playful St. Patrick’s Day twist.
Why You’ll Love This Recipe
- One pan with no layering
- Fluffy marshmallow frosting that you can eat with a spoon
- Festive green color that screams St. Patrick’s Day.
- Perfect for parties, school events, or family dessert.
- Easy to slice, serve, and snack on all day.
The Twist
Instead of a traditional buttercream, this cake is topped with a light marshmallow frosting that gives it a snack-cake vibe. It keeps the whole dessert themed way more fun than a standard frosted sheet cake.
Ingredient Highlights
- Sour cream: Keeps the cake soft, moist, and tender.
- Cornstarch: Helps create a lighter crumb.
- Marshmallow fluff: Gives the frosting that airy, nostalgic texture.
- Green food coloring: Adds the festive color without changing the flavor.
- Lucky Charms marshmallows: So perfectly themed
Lucky Marshmallow Sheet Cake
Ingredients
- 1 box vanilla cake mix. Optional: Pistachio cake mix can be used in place of vanilla if you can find it. It adds a subtle nutty flavor but isn’t required.
- Green food coloring
- 1 cup whole milk
- 1/2 cup vegetable oil
- 4 large eggs
- 1/2 cup sour cream
- 1 teaspoon vanilla extract
- 1 tablespoon cornstarch
- 1 cup salted butter, softened
- 2 cups powdered sugar
- 1 cup marshmallow fluff
- 1 teaspoon vanilla extract
- Rainbow sprinkles
- Shamrock sprinkles
- Lucky Charms marshmallows
Instructions
- Preheat the oven to 350°F. Line a 9 x 13 pan with parchment paper and lightly spray with baking spray.
- In a large bowl, combine the cake mix, milk, oil, eggs, sour cream, vanilla, cornstarch, and a few drops of green food coloring. Mix until smooth, about 30 seconds on low speed, then 1 minute on medium speed. Scrape down the sides as needed.
- Pour the batter into the prepared pan and spread evenly. Bake according to the box directions, checking a few minutes early. Let the cake cool completely.
- To make the frosting, beat the butter until pale and fluffy, about 2 minutes. Add the powdered sugar and beat until smooth. Mix in the marshmallow fluff and vanilla, then whip for one final minute until light and airy.
- Smooth the frosting evenly over the cooled cake. Sprinkle generously with rainbow and shamrock sprinkles, then arrange Lucky Charms marshmallows around the edges or scatter them over the top. Slice and serve.
FAQ
Can I use pistachio cake mix instead of vanilla?
Yes, if you can find it. It adds a subtle pistachio flavor but isn’t required.
Does this cake need to be refrigerated?
It’s fine at room temperature for several hours. Refrigerate leftovers and bring to room temp before serving.
Can I make this ahead of time?
Yes. Bake the cake a day ahead and frost it the day you plan to serve.
Can I skip the food coloring?
You can, but the green color is half the fun for St. Patrick’s Day.
What pan size works best?
A standard 9 x 13 pan is perfect for this recipe.
Sprinkle of Fun
Turn this into a full leprechaun party with green drinks, silly hats, and a few gold coins scattered on the table. This cake fits right in and disappears fast.
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