One cake mix will make about 12 cup cakes, so 2 mixes will probably do the job. Bake the cup cakes in muffin cups. When they are done, allow them to cool.
Use a soft whipped frosting and dip the tops in the frosting twisting as you pick it up to leave a swirl on top. Using a fudge frosting or cream cheese will probably require spreading frosting with a knife. If you are really dedicated you can use a cake decorating bag with a fancy tip. You can make two flavors of cake mix if you would like.
You might like to spell out a name or Happy Birthday using one cup cake for each letter, or use different colored sprinkles on the tops.
Look for recipes and ideas on the internet if this doesn't satisfy you. Good luck!