Spread some buttercream on the cake board.
Place one layer of cake on a cake board and place it in the fridge for 5-10 minutes. This will help the cake stick to the cake board and not slip as you stack and frost.
In the meantime, carve out circles in the middle of 2 of the cake layers for the candy filling
Remove from the cake and cake board from the fridge.
Top with approximately 1/3 cup of frosting and spread evenly.
Add one of the cake layers with the holes on top. Top with buttercream and spread evenly.
Repeat with the next cake layer.
Spread a thin layer of buttercream on the inside of the cake, to crumb coat where you will be filling the candy.
Place the cake in the fridge for 5-10 minutes.
While the cake is chilling, prepare the candies as required.
Remove cake from the fridge and fill the cavity with the prepared candies, and sprinkles. Fill the cavity completely without overfilling it.
Finish stacking the cake - Top with approximately 1/3 cup of frosting and spread evenly, then stack the final layer to hide the candy surprise.
Apply a thin coat of frosting all over the cake for the crumb coat.
Place cake in the fridge for 30 minutes.
Apply final base layer of white buttercream. Smooth as needed. Chill again for 30 minutes again.
Prepare pink and blue frosting - about 1/4 cup each.
Remove chilled cake.
Using an offset spatula, spread alternating blobs of blue and pink frosting.
Smooth gently with the bench scraper, wiping the scraper after each swipe, until desired look is achieved.
Fill a piping bag fitted with a 1M tip, with white vanilla Swiss Meringue Buttercream.
Create swirls on the top of the cake to finish the look.