Preheat oven to 350 F. Slice off all the rind from edges and sides of brie and discard. Cut cheese into small cubes and set aside.
Melt butter in a small saucepan over medium heat. Whisk in flour, mustard powder and cayenne.
Slowly pour in milk while stirring until combined. Heat, whisking constantly until slightly thickened, about 2 minutes. Add brie pieces and stir until combined and melted. Remove from heat.
In a medium bowl, lightly beat egg yolks. While whisking, slowly pour in cheese mixture until combined. Set aside to cool slightly.
Beat egg whites on medium in a stand or hand mixer until soft peaks form, about 3 minutes.
Scoop 1-cup of beaten egg whites onto cheese mixture and fold until combined. Pour this mixture back into mixing bowl and carefully fold with eggs whites until combined.
Pour into four medium or two large ramekin dishes, leaving about ¾-inch space from tops. Bake 30-40 minutes, or until top is browned and center jiggles slightly when shook. Serve immediately.
Use salted butter to enhance flavors
When making the butter and flour roux, stir often to prevent clumping
Same thing when adding milk and cheese to melt. A whisk works best for incorporating.
Add cheese mixture slowly to egg yolks, stirring constantly, so it mixes in well
Carefully fold mixture together just until combined to keep airy. It's delicate and too much mixing will deflate it.
Leave space in the dish when baking to allow for rising
It’s best to not open oven during baking as a change in temperature could cause souffle to drop. Instead turn on oven light to see. When it’s risen and you think it’s close to done, open oven quickly and see if it's golden brown.
Serve immediately as the souffle will drop while cooling