Prepare the ramekins: Preheat your oven to 375°F (190°C). Butter four 8-ounce ramekins (or two larger ones) generously. For sweet soufflés, sprinkle with sugar; for savory, use a little grated cheese.
- Make the soufflé base: In a medium saucepan, melt butter over medium heat. Add flour to make a roux, and cook for 1-2 minutes. Slowly whisk in milk and cook until thick. Remove from heat, and add egg yolks, vanilla (or mustard), salt, and nutmeg. Stir in cheese (or sugar for dessert soufflés) and set aside.
- Whisk the egg whites: Beat egg whites in a clean bowl with a pinch of salt until stiff peaks form.
- Combine the base and egg whites: Fold a small amount of egg whites into the soufflé base to lighten it. Then, gently fold the rest of the egg whites into the base until just combined.
- Fill the ramekins: Spoon the soufflé mixture into the prepared ramekins, filling them just below the rim. Smooth the tops and wipe away excess batter.
- Bake the soufflés: Place ramekins on a baking sheet and bake for 20-25 minutes, or until puffed and golden. Avoid opening the oven door during baking.
- Serve immediately: Soufflés deflate quickly, so serve them as soon as they come out of the oven. They can be enjoyed on their own or with a sauce of your choice.