Heat oven to 350 (or 325 for dark or nonstick pan). Spray bottom only of 13x9-inch pan with baking spray with flour.
As with most cake mixes, in large bowl, beat cake ingredients with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes. Pour into pan. You'll find this so very easy!
In large bowl, beat cream cheese, sugar, and 1 egg on medium speed until smooth and creamy. Stir in chocolate chips. Spoon filling by tablespoonfuls evenly over top of batter. Use a table knife to swirl filling through batter. Sprinkle with nuts.
Bake 40 to 45 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
The recipe calls for canned frosting, but I make my own, using the recipe on the 1 pound box of confectioners' sugar. I haven't tried chocolate frosting because I think that would be TOO much chocolate. It would be good, though, if you use a yellow cake mix.
Spread frosting over cooled cake. Store covered in refrigerator. (I never store in refrigerator. It doesn't last long enough to spoil!)