Coat 8 (6-ounce) ramekins with cooking spray; sprinkle with 2 tablespoons sugar. Place on a baking sheet; set aside.
Combine 1 1/2 cups milk, 3/4 cup sugar, cocoa, flour, and salt in a large saucepan; cook 5 minutes over medium-high heat, stirring constantly with a whisk until mixture thickens and comes to a boil. Cook an additional 30 seconds, stirring constantly. Remove from heat; add semisweet chocolate, stirring until melted. Gradually add chocolate mixture to egg yolks, stirring well. Return mixture to pan. Cook 2 minutes over medium heat, stirring constantly. Spoon mixture into a large bowl; cool to room temperature, stirring occasionally.
Beat egg whites and cream of tartar at high speed of a mixer until foamy. Add 1/3 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold one-fourth of egg white mixture into chocolate mixture; gently fold in remaining egg white mixture. Spoon batter evenly into prepared ramekins. Bake souffles at 400° for 20 minutes or until puffy and set.
Combine white chocolate and 2 tablespoons milk in a small saucepan; cook over low heat until chocolate melts. Remove from heat; stir in rum. Cut into each soufflé with a spoon; pour 1 tablespoon sauce over each soufflé. Serve immediately.