Bake 5 regular cupcakes and 36 mini cupcakes according to the recipe instructions; let cool completely.
Make your buttercream according to the recipe instructions (or use purchased frosting). Divide frosting evenly into three bowls. Add a toothpick tip' worth of gel food coloring to each bowl, mix to combine.
Fill your piping bag with your three colors as shown. Each of these colors complements the other, so if they run together, they'll form a pretty rainbow.
Tip: To make filling the piping bag easier, use a 32-ounce yogurt container. Set the piping bag into the container and use a spatula to fill the bag.
Your piping bag will look like this -- three colors in one! Squeeze out a bit of the frosting onto a piece of parchment paper or plate to get the colors to run together.
Now it's time to start piping the rosettes! Start in the center of your cupcake, holding the piping bag upright. Squeeze the piping bag and begin swirling in a counterclockwise motion. This creates the first rosette. Continue squeezing and do another rotation, pulling away the piping bag to make a single rosette. Pipe all of the regular and mini cupcakes in this manner and set aside.
Use the fondant to give your butterfly a face! Roll out your pink fondant and cut out a circle shape with a cookie cutter. Roll two small pieces of black fondant between your hands to make tiny eyes and do the same for antennae. Attach the antennae to toothpicks and make a tiny mouth.
Adhere the eyes and mouth to the pink circle with a tiny amount of water. (When fondant gets wet it gets sticky, so when the water dries it acts as a glue between the fondant.)
Place your pink happy face on top of one of your frosted regular cupcakes; insert the two toothpicks into the cupcake for antennae as shown.
Assemble the cake. You can do it on top of a large cake board, either prepurchased or homemade, or you can do this right on the party table that you will be serving dessert on. The body is simply 5 cupcakes (including the head) nestled together in a vertical line.
Now begin to add the mini cupcakes on the side. Make an arch shape on each side, like two mirrored rainbows, to create the tops of the wings. Position cupcakes in a diagnoal line to create the bottoms of the wings.
Keep going until you've filled in the wing shapes, using about 18 mini cupcakes on each side. Move them around and adjust until you find a butterfly shape that you love. Style your butterfly cake with heart-shape confetti, if desired, using pink paper and a crafts punch.