Sahnesteif is used to stabilize the whipped cream, preventing it from separating and going back to its liquid form and making it usable for frosting the cake. You might be able to find Sahnesteif as “Whip it” in some grocery stores or specialty bake shops and I’ve also seen it on Amazon. If you can’t find them, there are other ways to stabilize whipped cream, most notably using gelatine.