Preheat oven to 400 degrees. Butter dish well, and coat with superfine sugar. Cut a collar out of parchment paper to extend 3 inches above the rim of dish. Tie collar around outside of dish with kitchen string; using a pastry brush, butter the inside. Chill.
Chop chocolate, and place in a dry, heatproof bowl. Place over a pot of barely simmering water, and let stand until melted. Stir until smooth; keep warm.
Scald milk in a medium saucepan.
In a large bowl, beat yolks and 4 tablespoons sugar until pale and fluffy, using an electric mixer. Beat in flour until well combined. With mixer running, slowly add half of the hot milk.
Add mixture to saucepan with remaining milk. Bring to a boil over medium-high heat, stirring constantly. Simmer for 2 to 3 minutes more; whisk in melted chocolate. Transfer to a large bowl, and cover with plastic wrap, pressing it onto the surface. Keep warm until ready to use.
In a large bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add remaining 2 tablespoons sugar. Beat until stiff and glossy.
Spoon 1/4 of the whites into chocolate base, and whisk thoroughly until smooth. Quickly fold remaining whites into mixture until combined. (Don't worry if some streaks remain.) Transfer to prepared dish. Bake for 15 minutes at 400 degrees. Reduce heat to 375 degrees and bake 15 minutes more. Remove collar, dust with confectioners' sugar, if desired, and serve immediately.