Preheat oven to 350°. Stir together flour, sugar, cocoa, baking soda, and salt in 8×8-inch nonstick baking pan. Make 3 wells in dry mixture. Combine eggs and oil and pour into one well. Pour vinegar into second well. Pour vanilla into third well. Pour water over all and mix carefully with fork until all dry ingredients are moistened. Bake until top springs back when lightly touched, about 30 to 35 minutes. Cook and serve with Milk Chocolate Custard Sauce.
Milk Chocolate Custard Sauce: In small, heavy saucepan, stir together egg, sugar, salt and milk. Cook over low heat, stirring constantly, until mixture reaches 160° and is just thick enough to coat metal spoon with thin film, about 12 minutes. Remove from heat and immediately set pan in bowl of ice water. Stir in vanilla and chocolate until chocolate is melted. If serving warm, serve immediately and refrigerate any leftovers. To serve chilled, pour custard into bowl. To prevent skin from forming, press sheet of plastic wrap directly onto surface of custard. Chill thoroughly.