6 large egg yolks plus 8 large egg whites, room temp
6 Tbsp all-purpose flour
2 Tbsp brandy
1 tsp pure vanilla extract
Pinch cream of tartar
Preheat oven to 400 F, with rack in lower third. Do not open oven door until ready to bake. Brush outer lip of a 2-qt soufflé dish with melted butter. Tie a sheet of parchment around dish with kitchen twine so it extends 3 inches above rim. Brush inside of dish and collar with melted butter. Dust with sugar (this adds texture, which helps the soufflé climb); tap out excess. Chill dish in freezer 15 minutes. Stir chocolates in a large heatproof bowl set over a saucepan of simmering water until smooth. Scald the milk (heat it until it’s just about to simmer) in a saucepan over medium heat; remove from heat. Using an electric mixer fitted with the whisk, beat 6 Tbsp sugar and the yolks on high until pale, about 4 minutes. On low, beat in flour. Beat in half the hot milk, ladling it in a little at time (this is called tempering and prevents the yolks from scrambling).
Whisk mixture into pan of hot milk; bring to a boil over medium-high heat, whisking. Reduce heat to low; simmer until thick, about 2 minutes. Pour into chocolate. Stir in brandy, vanilla, and a pinch of salt. (This mixture can be refrigerated, 2 days. Rewarm in a heatproof bowl set over a pan of simmering water.) Egg whites are best whipped when they’re at room temperature (they attain more volume). Using a balloon whisk, beat whites and a pinch of salt in a copper bowl until foamy. (Or beat with an electric mixer in a stainless steel bowl with cream of tartar.) Add 1 Tbsp sugar; beat until whites almost hold stiff peaks. Add remaining Tbsp sugar, and beat until peaks are stiff (meaning they stand straight up when whisk is lifted).
Spoon one-third of egg whites onto base (incorporating in batches prevents them from deflating). Fold them in: Cut through center of mixture with a large rubber spatula, then gently turn spatula over. Rotate bowl a quarter turn; continue folding whites and turning bowl until mostly combined. Fold remaining whites into base, one-third at a time. The mixture is ready when the whites are fully incorporated. Pour mixture into prepared dish. Place on a rimmed baking sheet; this helps you move soufflé in and out of oven. Bake 15 minutes. Reduce temperature to 375 F; bake until set, 20 minutes. Remove collar; serve immediately with crème anglaise if desired.