Preheat the oven to 350 degrees F. Butter the ramekins and then dust with sugar. Place the ramekins in a 9-by-13-inch baking dish.
Place the chocolate chips and the butter in the top of a double boiler set over simmering water. Heat until melted, stirring occasionally so that the chocolate chips and butter are well blended and have a smooth, silky texture.
In a medium bowl, combine the cocoa powder, flour and salt. Mix together with a fork.
Using a hand-held mixer or a whisk, beat the egg whites in a large bowl until stiff peaks form.
In a medium bowl, beat the egg yolks and sugar with a hand-held mixer or a whisk until the mixture turns pale yellow, about 2 minutes. Slowly temper the melted chocolate into the egg yolks and mix until well combined. Mix in the milk. Add the dry ingredients and blend until fully incorporated. With a rubber spatula, fold in the egg whites in three separate additions.
Fill the prepared ramekins with batter two-thirds of the way up. Pour hot water into the baking dish two-thirds of the way up the sides of the ramekins. Cover the baking dish tightly with aluminum foil. Place in the oven and bake until the cakes are set, 15 to 20 minutes.
Carefully remove the ramekins from the baking dish and set aside to cool for about 30 minutes. Unmold the cakes onto individual plates. If the cakes are not releasing from the ramekins, run a small knife or offset metal spatula along the inside of the ramekins. Garnish each cake with a dusting of powdered sugar and a sprinkling of orange zest.