Preheat the oven to 190C with a baking tray inside.
Finely grate the hard cheese. Melt 15g of the butter in a medium saucepan and brush the inside of a 18 - 20cm soufflé dish with it. Scatter over the grated cheese to coat the sides, shake off any excess, and set the prepped dish aside.
Melt the rest of the butter in the same pan over a low to medium heat. Add the flour and whisk for a couple of minutes, then whisk in the milk, adding it slowly at first.
Turn the heat up a little and simmer for 3 - 4 minutes, whisking all the time, until the sauce thickens. Pour into a large mixing bowl. Crumble 125g of the soft cheese into the sauce and mix it in to melt. Whisk in the egg yolks, then season. Up to this point can be done ahead.
Put the egg whites into a separate and very clean mixing bowl. Whisk until they're stiff but not dry. You should be able to just about tip the bowl upside down without them falling. Chop the rest of the soft cheese into small pieces and crumble into the soufflé mix. Chop the herb leaves and mix those in, then gently fold in teh egg whites keeping as much air as possible - a few streaks of white are fine.
Put the dish into the oven immediately, onto the hot baking tray, and bake for 25 - 30 minuts. It's ready when golden, risen and with just a gentle wobble when you move the dish. Under no circumstances be tempted to open the oven door to check on it until it's been in for at least 20 minutes.
Serve straight away.