The last batch of brownies I made in the reflector oven got overdone...as they were cooking I asked "Do you think they should be BOILING in the pan?"  That was a bit TOO HOT, they cooked through and were tasty, but parts were the consistency of hard taffy when it cooled and took dynamite to pry them out of the pan.
You can regulate the cooking in your oven by where the fire is out in front of the oven.  Closer cooks the bottom of the pan more, further away cooks the top of the pan more.  SD is right about keeping a steady, even fire of moderate size...we make LOTS of kindling ahead of time to be able to feed the fire as needed with small wood.
If the top of what you are cooking is getting too brown ahead of time, try laying a pie tin, metal plate, or tin foil over the top while cooking longer to firm up the middle.  Do not over fill the cooking pan, keeping it a bit thinner will help cook the middle.  I also use a muffin pan and paper cups sometimes; the spaces between the muffin forms help transfer the heat and using paper cups makes it easy to remove the muffins when cooked (or while cooking, ahead of the rest if some are fully cooked and others are not)   ;)