Hello
This cake looks like it's covered in fondant, so a design like that could be put on by using a stencil.
You can stencil buttercream, but you have to know how to do it without making the buttercream stick to the stencil.
Fondant or marzipan would be best. Even a hard Royal Icing would be good.
There are also edible images that can be printed on "fondant paper". It works just like a regular table top printer, except the ink is made from food coloring and the paper is made from fondant. You can print any image you want, any size you want from your computer to the image printer. You can order these custom printed online also.
You can look up some videos on YouTube if you want to see how this works.
If you use this method, you can use any kind of frosting or cake covering, as the fondant is peeled off the paper backing and placed onto the area of the cake where you want the decoration.
Depending on the look you want, you can use a stencil along with either an airbrush, paint brush, sponge, or food coloring pens to color in the stencil image.
You can also use an ink stamper, where the ink you use is food coloring rolled onto the stamper, and then stamping the area on the cake you want decorated. In order to do this, you must use either fondant or marzipan for cake covering. You have to make sure your cake is firm and flat, so when you press the stamper onto the cake, it doesn't push in or cave in, or dent the fondant or marzipan.
Or, if you are the Michael Angelo type, you can spend a few hours with a paint brush and some food coloring, doing it all by hand.