I've been slicing cakes this way for ages. The cake in the photo had lots of fruit on top, which I had to remove before slicing. That's why the whipped cream looks a little jaggy. Some cake may have a really soft sponge and cream, but you only need to warm your knife to get nice results.

Wipe the cream off of your knife before each slice. If your cake has hard cream, you can heat your knife over a gas flame instead. If your cake is soft, the whipped cream will melt a little when you slice it right away. To prevent melting, let the knife cool a bit before slicing. Recipe by Yuuyuu0221