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Thomas the Train Smash Cake

Thomas the Train Smash Cake

This Thomas the Train Smash Cake is decorated with Easy Vanilla Buttercream and homemade Marshmallow Fondant.
5 from 9 votes
Prep Time 55 mins
Cook Time 5 mins
Rest Time: 2 hrs
Total Time 3 hrs
Course Dessert
Cuisine American
Servings 6


For the Marshmallow Fondant:

  • vegetable shortening (for greasing bowls)
  • 8 ounces plain marshmallows (1/2 bag)
  • 1 tablespoon water
  • 1 teaspoon clear flavoring (such as vanilla extract)
  • 1 pound powdered sugar (divided)
  • pinch of salt

For the Cake:

  • 1 box cake mix of your choice (prepared and baked in 2 or 3 6-inch layers)
  • 1 batch Easy Vanilla Buttercream (see note)

For Decorating:

  • blue, red and black food coloring gel
  • black gel icing (for drawing train track)


Make the Marshmallow Fondant:

  • Using the shortening, generously grease a large microwave-safe bowl, the bowl of the stand mixer, a spoon/spatula (or two), and the mixer dough hook.
  • Add the marshmallows and water to the prepared microwave-safe bowl and microwave for 60 seconds. Stir with the greased spatula/spoon. If not all the marshmallows have melted, microwave for 30 more seconds. Stir in the flavoring.
  • Reserve 1/2 cup of the confectioners’ sugar and add the rest to the bowl of the stand mixer, along with the salt. Create a well in the center. Pour the marshmallow mixture into the well and turn the mixer on to the lowest setting. When the mixer sounds strained, turn the speed up one setting. Turn off the mixer once all the sugar has been incorporated. If the fondant is sticky, add the reserved sugar 2 tablespoons at a time until it is no longer sticky. You may not need any.
  • Turn the fondant out onto a piece of plastic wrap. Rub a bit of shortening on the outside of the ball. Wrap tightly with the plastic wrap, place it in a resealable plastic bag and let rest for at least 2-3 hours. Keep the unused portions covered when not in use. If the fondant becomes stiff, microwave for 20 seconds at a time until it becomes pliable.

Assemble the Cake:

  • Cover a cake board or platter with aluminum foil.
  • Place a small amount of buttercream in the center of the cake board and place 1 layer of cake on top. Cover layer with buttercream and add another layer of cake. Repeat for all cake layers and cover cake with a thin crumb coat. Refrigerate for 15 minutes.
  • While the crumb coat sets, color portions of the fondant dark blue, red and gray. Roll the fondant out and use a sharp knife to cut out train, wheels, and a "1" shape. You will likely need 3 train shapes, 6 smaller wheels, 3 larger wheels, and 4 "1" shapes.
    Thomas the Train Smash Cake
  • Tint buttercream with blue food coloring gel to a medium blue. Tint a small amount of buttercream to dark blue for the lettering.
  • Frost the outside of the entire cake with the medium blue frosting. Apply the train, wheels, and 1's around the cake and place the last "1" on top of the cake.
  • Fill a pastry bag with a small round tip and pipe the lettering on top with the dark blue buttercream.
  • Fill a pastry bag with a small star tip and pipe a border around the top and bottom edges of the cake.
  • To finish, pipe a train track around the cake on the cake plate with the black gel icing.


Marshmallow Fondant recipe is a half batch 
Fondant Source: Adapted from Annie's Eats
Easy Vanilla Buttercream recipe
You're Gonna Bake It After All
Keyword American, cake, dessert, party