We did the same for our daughter when she turned one.
Here is a simple recipe that I used...
1 box cake mix
make according to directions using 2 round pans
16 oz cool whip
1 package of mini oreos
1 cup fruity pebbles
food coloring
Bake the cake according to the directions using 2 round cake pans. Cool completely.
While cakes are cooling, color cool whip with your choice of food coloring.
Cut cakes in half and place on cookie sheet with round sides facing in to make wings. It will make a 2 layer cake.
Layer and frost cakes. Sprinkle with fruity pebbles.
Place oreos randomly on butterfly.
Stack oreos in between butterfly wings...place a dollop of cool whip in between oreos so they stay put.
Refrigerate until ready or cool whip will melt.
You could use regular white frosting instead of cool whip.
If you have any questions let me know. I can search for the actual recipe...I kind of went by memory.
T.