Heat oven to 350°F. Grease and flour three 8- or 9-inch round cake pans. In medium bowl, beat 1/2 cup butter and 1 1/4 cups granulated sugar with electric mixer on medium speed, scraping bowl occasionally, until blended. Beat in 2 eggs, one at a time. Beat in chocolate, 3/4 cup milk and 1 teaspoon vanilla. Beat in remaining chocolate cake ingredients on low speed, scraping bowl constantly. Divide batter evenly between 2 pans.