How to get a RICH chocolate cake from a cake mix?!

You will never acheive what you are looking for out of a box in that a 5 star restaruant uses first rate ingredients that you are not going to find in a box.

That said, I find that if I add a couple of heaping tablespoons of coco powder and vanilla to my boxed brownie mix, they come out GREAT so you may try it with a cake mix.

Honestly, that'll be a little difficult to achieve from a boxed cake mix. Restaurants either make their cakes from scratch themselves or get them someplace that does make it from scratch themselves. Rich chocolate cakes usually use more than one kind of chocolate (cocoa powder, melted chocolate, etc...) for their batter. If you want to achieve a richer chocolate cake from a mix, you may want to try either adding cocoa powder or melted chocolate to your mix or even using a brownie mix instead and modifying it for a cake which will give you the richness but still not be quite right.

is you pre-mix cake is a chocolate cake or just a normal butter cake? if it is chocolate, add extra chocolate by double-boiled dark chocolate (cut into pieces) and mix with the other pre-prepared ingredients. or add extra chocolate chips or chocolate chunks and mix together with the other ingredients (inside and/or top of the cake)

I used to work with a woman who made WONDERFUL cakes that I though were from scratch. She told me one day that she uses box mixes and always uses buttermilk instead of water and an extra egg.
For chocolate cake, I would maybe use 1/2 the amount of water called for and substitute mayo (not salad dressing) for the other portion of liquid or maybe even sour cream. Coffee actually makes chocolate cake wonderful, so you could try using brewed coffee instead of the water and 1/4 teaspoon cinnamon. I have done both of these things and they make for a moist, flavorful cake.

Take the cake mix and instead of adding oil add melted butter and a box of instant dark chocolate pudding mix. Instead of water add cream or milk and then the recommended amount of eggs.

Good luck!

You could probably add some milk chocolate into the batter.

Add an extra egg or two.