Instructions

1. Use 1 teaspoon of the butter to grease the inside of a 1½-quart baking or soufflé dish. Sprinkle with the grated Parmesan and rotate the dish to coat the inside. Store in the refrigerator until ready to use.
2. In a medium saucepan, melt 2 tablespoons butter over medium heat. Add the flour and whisk to form a smooth paste. Slowly add the milk, whisking constantly to avoid lumps. Bring to a simmer and remove from the heat. Whisk in the mixed cheese, cayenne, and salt until smooth.
3. Meanwhile, whisk the 3 egg yolks in a medium bowl. Whisk in the warm sauce, a little at a time to avoid cooking the egg. Whisk constantly until all of the warm mixture is mixed into the egg. Set aside to cool to room temperature. Preheat the oven to 375°F.
4. Using a hand mixer, beat the 4 egg whites and the cream of tartar on high speed until stiff peaks form. They should be opaque and shiny, not dry. Fold 1/4 of the whites into the cheese mixture to lighten slightly. Fold the remaining whites into the cheese mixture in 2 batches, being careful not to overmix and deflate the whites, it's okay if there are a few remaining streaks.
5. Pour the mixture into the prepared dish and place the dish on a rimmed baking sheet. Place the tray in the preheated oven for 25 to 30 minutes or until puffed and golden. It should be firm but still jiggle slightly. Serve immediately.