3 tbsp. unsalted butter
1 shallot, finely chopped
2 tbsp. flour
1 1⁄2 cups milk
4 oz. grated sharp white cheddar
1⁄2 tsp. mustard powder
1⁄8 tsp. cayenne
1⁄8 tsp. ground nutmeg
6 eggs, separated
Kosher salt and freshly ground black pepper, to taste
4 oz. finely chopped ham (any variety will do)
1 1⁄2 tbsp. finely chopped chives
1⁄4 cup finely grated Parmesan
1. In a 2-qt. saucepan, melt 2 tbsp. of the butter over medium-high heat. Add the shallots; cook until soft, 1-2 minutes. Whisk in the flour to make a smooth paste; slowly add the milk while whisking. Bring the sauce to a simmer, stirring, until thickened, about 5 minutes. Remove from heat; stir in the cheddar, mustard, cayenne, nutmeg, egg yolks, and salt and pepper. Stir over medium heat until just steaming, 3-4 minutes. Transfer the cheese mixture to a large bowl; stir in ham and chives; chill for 30 minutes.
2. Heat oven to 375°. Grease six 8-oz. ramekins with the remaining butter. Coat the bottom and sides of ramekins with Parmesan; shake out any excess. In a large bowl, whip the egg whites to stiff peaks. Working in 3 batches, fold egg whites into the chilled cheese mixture to make a fluffy batter. Gently divide the batter between prepared ramekins and transfer to a rimmed baking sheet. With a damp cloth, wipe away any of the soufflé batter that's splashed onto the ramekin rims. Bake until golden brown and puffed, about 30 minutes. Serve immediately.