Fill a piping bag fitted with a large french star piping tip with buttercream, then set aside.
With the remaining buttercream, gently spread on a flat layer with an offset icing spatula and smooth it flat with a flat cake scraper.
Using a stripe cake scraper with indents, create clean indents in the buttercream. If it doesn't look clean (meaning, clear lines), add on some buttercream in those flawed areas, and scrape again. Clean lines will equal clean stripes, so this step is essential to get right. Be patient, and those lines will appear!
Freeze the cake for 15 minutes, until the buttercream is firm. Make the ganache.
Pipe in the ganache into the clean indents, and tap the cake flat after each one to spread the ganache into the indent to create a stripe. Repeat with the remaining stripe indents.
Pipe line of shells on the two short ends of the cake, then in the center, then between each one so you end up with 5 lines of shell borders.
Sprinkle on the chocolate shavings, and enjoy!