Preheat your oven to 375°F (190°C). Generously grease four 6-ounce ramekins with 1 tablespoon of butter, then coat the insides with a little grated Parmesan cheese, shaking out the excess.

In a medium saucepan over medium heat, melt the remaining 2 tablespoons of butter. Whisk in the flour and cook for 1 minute to create a roux, but do not let it brown.

Gradually whisk in the milk until smooth. Simmer for 2-3 minutes, stirring constantly, until the mixture thickens into a béchamel sauce.

Remove the pan from the heat. Whisk in the Gruyère, Cheddar, 3 tablespoons of Parmesan, salt, pepper, chives, and thyme until the cheese is melted and smooth.

Whisk the egg yolks into the warm cheese mixture one at a time until fully incorporated. Set aside to cool slightly.

In a clean, dry bowl, beat the egg whites with the cream of tartar using an electric mixer until stiff, glossy peaks form.

Gently fold one-third of the egg whites into the cheese mixture to lighten it. Then, carefully fold in the remaining egg whites until just combined, being careful not to deflate the batter.

Divide the mixture evenly among the prepared ramekins. Run your thumb around the inside rim of each ramekin to create a small channel (this helps them rise evenly).

Bake for 20-25 minutes, or until the soufflés are puffed and golden brown on top. Do not open the oven door during the first 15 minutes.

Serve immediately while hot and fluffy.