Preheat oven to 400°F. Butter four 1-cup ramekins, coat with Parmesan, and refrigerate.

Melt butter in a saucepan over medium heat. Add flour and stir for 1 minute. Add milk, whisking until thickened, 2 minutes. Remove from heat and transfer mixture to a large bowl. Whisk in Emmental, rosemary, salt and pepper. Whisk in egg yolks.

In a clean bowl, beat egg whites until stiff peaks form. Fold into cheese mixture in thirds.

Divide the mixture between ramekins. Place the ramekins in a baking tray filled with enough hot water to reach about halfway up the side of the dish. Bake until puffed, approximately 20 minutes. Top with bacon slices and serve immediately.