Buttercream recipe URGENT!

It's my SO's birthday tomorrow and I am baking him a cake. He specifically wants a chocolate cake covered with piped rosettes.

I bake almost professionally (worked in a bakery), but the cakes we have in our country don't use buttercream!

Do you have a 100% chocolate buttercream recipe that will hold its shape? In this case the piped rosettes.

I can't afford to mess it up. I know there are a milion recipes, and it's easy, but I want a trusted recipe. Thank you!