You don't need to buy a special pan to make a butterfly cake - it is simple to create one from a standard 13x9 sized pan. This can easily work with square or round cakes also (see the tips).
After the cake has cooled, remove it from the pan. Lining the pan with parchment before baking or having the pan well greased will make for easy removal. To prevent breaking when removing the cake, set a solid surface (e.g. cutting board) on top of the cake, and holding it in place, flip the pan upside down and set it on the counter. Gravity will do its thing and the cake will eventually loosen and drop onto the cutting board.
Turn the cake so it is wider than it is tall, and cut one strip about 1.5" wide off of the left or right side and move it out of the way. With the remaining portion of the cake, make two cuts that form an X going from corner to corner and gives you four triangles.
To build the butterfly, set the small cake strip in the center as a body. On each side of the body, place two triangles so a point of each is touching the body and they fan out to look like wings. Cut off any sharp corners you'd rather not have and do any additional shaping.
Ice and decorate. Icing the 'cut' edges of the butterfly can be difficult - go slow and don't use too much pressure. To prevent icing from getting on the presentation surface, slide pieces of parchment paper or foil under the edges of the cake so they can be pulled away afterwards.
When arranging the wings, try to put the uncut edges facing outwards, as they are easier to ice and have a more finished look. For square cakes you can follow the same cutting pattern or you may just cut the body and then cut the remaining cake into two large pieces for wings For round cakes, cut the body from the center of the cake, leaving you with two large rounded wings. You may cut those in half also so each wing will be made up of two segments.