Preheat the oven to 400 degrees F. Combine milk, sugar and vanilla in a heavy, non-reactive saucepan and bring to a boil. Meanwhile, sift together cocoa and cornstarch. Remove boiled milk from heat. In a steady stream, whisk dry ingredients into hot milk. Stirring constantly, continue cooking until base is thickened and returns to a boil. Strain through a chinois or fine mesh strainer and chill in an ice-water bath. Meanwhile, place the egg whites in the bowl of an electric mixer fitted with the whisk attachment. Begin whipping the whites, gradually adding the sugar, until soft peaks are achieved. Gently fold the egg whites into 1/4 cup of the base, and divide among the ramekins. Tap the ramekins to release any large air pockets, and smooth the tops with a spatula. Transfer to the oven and bake for approximately 8 minutes, or until fully risen. Dust with powdered sugar and serve immediately.