What makes the difference between domed and flat cookies?
Baking powder instead of baking soda could certainly cause that dome. Baking soda requires an acid to leaven, baking powder has that acid included. Cookies don't have a lot of acid on their own so baking soda doesn't cause them to rise much. 

The baking soda is in a recipe like that to help with browning by altering the PH, soda makes the dough more alkaline. Since baking powder includes an acid, it doesn't have the same effect on the PH of the dough. That's probably why your cookies aren't as nicely brown as the ones in the other picture.

Try using the baking soda next time, let us know how it it goes.

Concerning your butter, you might like what melting it does, see Why is there a difference between softened and melted butter when baking?, but I'd definitely recommend trying the baking soda first.