Preheat oven to 375oF (190oC). Generously brush the inside of 4 mini cocotte dishes with butter.
In a large bowl mix together the mashed potato, butter, grated cheese, half and half, and rosemary. Stir in the egg yolks and season with salt and pepper.
Add the egg whites to a large bowl. Whisk until the egg whites form stiff peaks.
Add a heaping scoop of egg whites to the potato mixture; gently stir to combine. Fold in the remaining egg whites until no streaks of white remain.
Fill each of the prepared soufflé dishes 3/4 of the way full with the potato mixture.
Bake for 30-35 minutes, or until the soufflé is puffed and a rich golden brown color. Serve immediately.