1Heat oven to 350°F (325°F for dark or nonstick pan). Generously spray bottom only of 8- or 9-inch square pan with cooking spray (without flour).
2In large bowl, beat cake mix, buttermilk, food color, baking soda, butter and eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour batter into pan.
3Bake 40 to 45 minutes or until toothpick inserted in center comes out clean. Cool on cooling rack 10 minutes. Run knife around edge of pan to loosen cake; remove from pan to cooling rack. Cool completely.
4Place cake on serving plate. Frost top and sides of cake with cream cheese frosting; sprinkle with cocoa. Cut into 4 rows by 3 rows.