Begin by baking your four cakes in 9" square cake pans following the instructions on the box.
Allow them to cool completely, then flatten each of the cake layers by trimming off rounded tops. Do not worry about trimming the rest of the cake in order to get sharper corners right now.
Next, cut a square out from the very center of two of the cake layers. You don't need any special tool to do this. Do you know what I used? A metal spatula. The holes do not have to be perfect either, just make sure both layers have the same size hole. These two cakes will be the two middle layers of the piñata cake.
Place one layer on a square cake board and frost it lightly. This will be the bottom of the cake. Don't make the frosting too thick or the layers could slip and slide.
Now place one of the cake layers with a hole on top of that bottom frosted layer.
Lightly frost the top of it.
Then add the next layer with the hole in it (this is the third layer of the piñata cake).
Once you have that third layer on, and before you frost it, grab the M&M's® Holiday Milk Chocolate candies and fill the hole up to the very top with the candies.
Once you fill the hole, lightly frost the top of that layer, then add the final layer as the top. Make sure you flip this layer upside down so you have a smooth surface to frost!
Now, carefully place the entire cake in the fridge for a couple of hours. This helps a lot when it comes time to cut the cake into a more square-like shape, the cake won’t rip as badly so you will get a nice clean-cut. Once the cake is thoroughly chilled, trim it into shape, using a bread knife.
Next, apply a crumb layer (a thin layer of frosting that helps give a nice smooth surface to frost) to the top of the cake and all sides.
Be very careful when icing, especially around the corners! As you can see in the pictures I had some trouble with mine, but once I added the final layer of icing, and all of the decorations, you really could not tell.
Place the cake back in the fridge to chill until the frosting is firm to the touch (about 30 to 45 minutes). Once it is thoroughly chilled, the cake is ready to be iced with a thick second layer of frosting. Try to get the frosting as smooth as you can, but don't worry about making it perfect.
Don't forget to add those festive sprinkles from the Pillsbury™ Funfetti® Holiday Vanilla Flavored Frosting!
Make a simple ribbon from green fondant.
Use some M&M’s® White Peppermint candies for the bow.
Then add a line of the candies around the bottom edge of the cake.