I don't know of any other recipe but this one - mix a can of diet soda with any cake mix, put into a greased and floured pan, bake in a 9x13" pan at 325º for about 25 minutes - or until it's done the way you like it.
Can frost or just dust with powdered sugar.
Good combinations: diet coke with chocolate or fudge,
diet Sunkist with fudge chocolate, yellow, lemon, white or orange cake,
diet Dr Pepper with chocolate, fudge, white, yellow or spice cake