Use high-quality chocolate -- look for four-ounce bars in the candy section of the supermarket.
Heat oven to 400 degrees. Butter dish well. Coat with sugar and tap out excess. From baking parchment, cut a collar to extend 3 inches above the rim of dish. Tie collar around dish with kitchen string, then, 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 the 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 the hot milk.
Add mixture to saucepan with remaining milk. Bring to a boil over medium-low 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 a third 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 toprepared dish. Bake for 15 minutes at 400 degrees, then reduce heat to 375 degrees and bake15 minutes more. Remove collar, dust with confectioners' sugar or cocoa, if desired, and serve immediately.