Position a rack in the center of the oven and preheat to 350 degrees F. Butter and flour a 9-inch springform pan, and tap out the excess flour.
Melt the chocolate in the top part of a double boiler over hot, not simmering water. Remove from the heat and let cool until tepid.
Sift together the flour, cornstarch, baking powder, baking soda, and salt. Using an electric mixer at high speed, beat the sugar and butter in a large bowl until the mixture is light and fluffy, about 3 minutes. Gradually beat in the eggs, then beat in the melted chocolate and vanilla. Reduce the mixer speed to low. Starting with the flour mixture, add three equal additions of the flour, alternating with two equal additions of the milk, beating until smooth and scraping down the sides of the bowl after each addition. Spread the batter in the pan.
Bake until a wooden toothpick inserted in the center of the cake comes out clean, 45 to 50 minutes. Let cool in the pan on a wire cake rack.
Run a sharp knife around the inside of the pan to loosen the cake from the sides of the pan. Release the sides of the pan. Transfer the cake, keeping it on the pan bottom, to a serving platter. Spread the top of the cake with 1 cup of dulce de leche. Slice and serve. Drizzle the plate with additional dulce de leche.
Makes 12 servings