Hot Chocolate Soufflé

1/2 cup sugar, DIVIDED
1/3 cup unsweetened cocoa powder
1/4 cup all-purpose flour
1/8 tsp. salt
1 cup milk
1/2 tsp. vanilla
4 EGG WHITES, room temperature
1/4 tsp. cream of tartar
4 EGG YOLKS
Here’s How

HEAT oven to 350°F. MIX 1/4 cup sugar, the cocoa powder, flour and salt in medium saucepan; gradually whisk in milk until smooth. COOK over medium heat, stirring constantly, until mixture thickens and boils. STIR IN vanilla. Remove from heat.
BEAT egg whites and cream of tartar in mixer bowl with whisk attachment on high speed until foamy. Beating constantly, ADD remaining 1/4 cup sugar, 2 Tbsp. at a time, beating after each addition until sugar is dissolved. (Rub a bit of mixture between thumb and forefinger; it should feel completely smooth.) Continue beating until whites are glossy and stand in Soft Peaks.
STIR egg yolks into reserved sauce until blended. Gently but thoroughly FOLD yolk mixture into whites until no streaks of white remain. Carefully POUR into ungreased 1-1/2 to 2-qt. soufflé dish. For a "top hat", hold metal spatula upright and make a ring in top of the soufflé mixture, 1 inch from side of dish and 1 inch deep, if desired.
BAKE in 350°F oven until soufflé is puffy, delicately browned and shakes slightly when oven rack is moved gently back and forth, 30 to 40 minutes. SERVE IMMEDIATELY.