Preheat the oven to 350 degrees.

Prepare a 1-quart soufflé mold by lightly buttering it, dusting the insides with sugar, and tapping out the excess. Place the mold in the refrigerator.

Pour the milk, cocoa powder and sugar into a heavy saucepan and stir to combine. Bring to a boil over moderate heat, stirring constantly. Reduce the heat and cook, still stirring, until the mixture thickens, about 10 minutes. Transfer to a bowl and cool slightly.

Stir the egg yolks into the warm chocolate mixture. Stir in the butter.

In a separate bowl, beat the egg whites until they reach soft peaks. Add the salt and beat until stiff. Whisk half of the egg whites mixture into the chocolate mixture. Fold in the remaining egg whites gently with a spatula. Pour the mixture into the soufflé mold and smooth the top.

Bake on the lower-middle shelf of the preheated oven until puffy and brown, about 18 minutes, which will give you a soft center. Serve at once with whipped cream.