Beat the butter and the sugar until fluffy, then add the egg yolks one by one, until a smooth, foamy cream forms.
Break the chocolate into small pieces, melt it with 2 tablespoons of water in a saucepan over low heat, stir smooth, then let cool completely.
Stir the cooled chocolate into the egg-and-butter cream, and mix thoroughly.
Whip the 6 egg whites into a stiff foam, and fold gently into the cream.
Pour the batter into a buttered, floured mold.
In a larger pot, bring enough water to a boil to stand about two fingers deep; set the uncovered mold into the already-boiling water, cover the pot, and steam over low heat for about 45 minutes, until the soufflé has risen and the center is cooked through.
Remove carefully from the water, let cool for a few minutes in the mold, then turn out onto a plate and serve immediately.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.