Fold the butter and mascarpone together until smooth (if the mixture seems too thick to easily combine, microwave for ten seconds at a time until just loose enough to stir).
Spread onto a marble pastry board, then top with sprigs of rosemary, finishing salt, and edible flowers.
Serve with a loaf of good bakery bread, like a baguette or boule. I prefer to let guests rip off pieces by hand (I'm really into hands on serving), but you can slice in advance if you prefer a more polished experience.
Optional but encouraged: serve with a honey pot and dipper so guests can drizzle a bit of honey over their slice -- pairs wonderfully with the rosemary.