Start by preheating your oven to 375°F (190°C). While that heats up, grease a 1 ½-quart soufflé dish with softened unsalted butter. Dust a little flour inside the dish to help the soufflé climb. This will create perfect soufflé walls that will hold your creation as it rises.

In a saucepan over medium heat, melt the 3 tablespoons of unsalted butter. Add the flour and whisk for about 1 minute until combined. Gradually pour in the chilled milk, stirring continuously to prevent lumps. Cook for around 2-3 minutes until the mixture thickens. Remove from the heat and season with salt, pepper, nutmeg, and Dijon mustard.

To the warm mixture, stir in the grated Parmigiano-Reggiano and sharp Cheddar cheese until melted and smooth. Next, add the egg yolks and fresh chives, mixing well to incorporate. Set this mixture aside to cool slightly.

In a clean mixing bowl, use an electric mixer to whip the 5 egg whites with cream of tartar until stiff peaks form. This process is the magic ingredient that will give your soufflé its wonderful rise.

Carefully fold the whipped egg whites into the cheese base. Start with a small portion of egg whites to lighten the mixture. Then, gently incorporate the rest, being cautious not to deflate the whites.

Pour the combined mixture into your prepared soufflé dish, filling it about three-quarters full. Place it on the middle rack of the preheated oven. Bake for about 25-30 minutes or until it’s golden brown and puffed up.

Remove the soufflé from the oven. It will start to deflate quickly, so serve it right away to showcase that beautiful height!