1. Preheat oven to 350°. Stir together cornstarch and 1/2 cup sugar. Toss berries with cornstarch mixture, and spoon into a lightly greased 11- x 7-inch baking dish.
2. Beat butter at medium speed with an electric mixer until fluffy; gradually add remaining 3/4 cup sugar, beating well. Add eggs, 1 at a time, beating just until blended after each addition. Stir in red food coloring and vanilla until blended.
3. Combine flour, cocoa, and salt. Stir together buttermilk, vinegar, and baking soda in a 2-cup liquid measuring cup. (Mixture will bubble.) Add flour mixture to butter mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Spoon batter over berry mixture.
4. Bake at 350° for 45 to 50 minutes or until a wooden pick inserted in center of cake topping comes out clean. Cool on a wire rack 10 minutes.
Switch It Up
Chocolate-Cherry Cobbler: Omit red food coloring. Substitute 6 cups pitted fresh cherries (about 2 1/2 to 3 lb.) for berries. Decrease flour to 1 cup, and increase unsweetened cocoa to 1/4 cup. Proceed with recipe as directed. For a Chocolate-Cherry Cobbler sundae, top scoops of Chocolate-Cherry Cobbler with vanilla ice cream, Hot Fudge Sauce, sweetened whipped cream, and fresh cherries (with stems).