If you’d like to enhance the chocolate flavor even more you can add a bit of cocoa powder to the buttercream like I did here in my German Chocolate Cake. You can just add it 1 Tbsp at a time until you get the desired flavor or color.
If you want a darker chocolate frosting you can also add some chocolate brown color gel to it. I don’t find it necessary, but it’s an option.

If you prefer an easier frosting, I have a great recipe for an American-style easy chocolate buttercream that you can use instead. Or, if you want something even richer, you can pair it with a chocolate ganache frosting.