Preheat the oven to 350F. Grease and flour an 8 or 9-inch cake pan. In the top of a double boiler melt the chocolate with the milk over barely simmering water. In a mixing bowl cream together the butter and sugar. Add the eggs, one at a time, and beat well. Stir in the vanilla and the cornstarch. Pour in the melted chocolate and stir to mix well. Pour into the prepared pan. Bake for 20-25 minutes, until a knife comes out clean when inserted in the center. Cool on a rack. Whip the cream, adding the sugar, 1 tablespoon at a time once it has reached the soft-peak stage. Slice the cake in half horizontally. Place the lower half of the cake on a cake platter and spread it with the whipped cream. Replace the top half of the cake. Melt the sugar in 2 tablespoons of water over low heat stirring occasionally. As soon as the sugar melts, raise the temperature and boil hard until the temperature registers 239 F on a candy thermometer. Chop the chocolate pieces small and stir them into the syrup gently. Cool the syrup to 120F, then pour it evenly over the cake. Refrigerate for a minimum of 3 hours. Serve with the custard sauce.
