Separate out about a cup and a half of frosting, and color it bright orange. Place it in a piping bag fitted with a large round tip or coupler.
Divide the remaining frosting in half, and color one half bright green and one half bright purple. Place each of these colors in piping bags as well.
Place the 6-inch cakes on 6-inch cake cardboards. Trim the tops of the cakes so that they're level, and cut each cake in half so you end up with four cake layers total. Place one cake on a turntable, and pipe a thick circle of green frosting along the outside edge. Pipe a thick orange circle inside of the green one, then add a purple ring inside that. Continue to pipe circles of buttercream in the same pattern until you reach the center of the cake.
Add a second cake round on top, and repeat the frosting circles. Add a third cake round and repeat once more. Place the last cake round on top, press down gently, and make sure the sides of the cake are straight. Spread a very thin layer of green frosting along the top and sides of the cake to lock in any crumbs. Refrigerate for 45-60 minutes, until the cake is firm.
While you wait for the 6-inch cake to set, prepare the 4-inch cake. Place it on a 4-inch cake cardboard and cut the cake in half. Do not cut off the top to make it level—we want that cake dome!
Repeat the frosting circles inside the cake, but this time start with purple along the outside edge. Add the top of the cake, and cover it with a thin layer of purple frosting to lock in crumbs. Refrigerate until firm.
To decorate the 6-inch cake, remove it from the refrigerator once it is firm. Spread more green frosting on the top and sides of the cake. Use a metal spatula to make it as smooth and even as possible, but don’t worry about getting a perfect finish because it will all be covered by sprinkles. Take a 4-inch round cookie cutter, and center it on top of your cake, embedding it slightly in the top.
Place the sprinkles in a bowl, and hold the cake in one hand, over a rimmed baking sheet. Take a handful of sprinkles in the other hand, and press them gently but firmly against the side of the cake. Let the excess sprinkles fall into the pan, so you can scoop them up and re-use them. Cover the sides of the cake entirely. Cover the top of the but leave the center of the cookie cutter empty. (Leaving the blank space in the middle will make it easier to layer the cakes.)
Once the cake is covered with sprinkles, take a dowel and insert it into the center of the cake. Trim the dowel so it’s flush with the top of the cake, then spread a thin layer of frosting over it to adhere the top layer. Carefully set the purple monster cake on top, centering it over the dowel and making sure it’s secure.
Fit a grass (multi-opening) tip onto the bag of purple frosting. Pipe long strands of fur all over the monster cake. I like to begin about an inch from the bottom and pull the strands down so it’s like shaggy fur. After you’ve done the bottom layer, go up another inch and add a second layer that overlaps the first. Continue until you’ve covered the entire cake with frosting fur.
You can decorate the monster with whatever candies and treats you have available! To make yours look like mine, twist open 3 Oreo thins, and press a mini M&M into the center of each cream-topped cookie.
Roll out black fondant, and cut it into a smiling mouth shape. Roll out a thin tube of white fondant, and cut it into small triangles to make teeth. Stick the teeth onto the black mouth with a little dab of water on each tooth.
Take 2 green fruit chews, and roll them between your hands to make horn shapes. Press a toothpick into the base of each horn, so they’re easier to secure into the cake. Take 2 more green fruit chews, and flatten them out. Cut them into a foot shape. Take 2 orange fruit chews, and roll them into balls. Skewer them on lollipop sticks, and roll them in orange sparkling sugar.
Now assemble! Press the eyes, mouth, and feet onto the front of the monster. Stick the horns securely into the sides of the monster, and insert the lollipop sticks halfway into the top of the head, so the antennae stick out.
For the best taste and texture, serve this cake at room temperature. Leftovers can be stored, well-wrapped, in the refrigerator for up to 5-7 days. The candy elements might start to get sticky and unappetizing, so keep an eye on them and remove them if necessary.