Heat the mixture over a medium heat, stirring constantly with a heatproof spatular until the butter melts and the mixture lightens in color.
Add the vanilla and cocoa powder to the pan and stir until they are incorporated into the butter mixture.
Add the eggs and mix until the batter looks shiny.
Add the flour and baking powder. Stir until the dry ingredients are incorporated into the batter.
Stir in the chips and nuts making sure to reserve a few of the walnuts to add to the top of the brownies right before you bake them.
Pour the brownie mixture into the cast iron muffin pan making sure it spreads to the edges.
Sprinkle the remaining walnuts on top of the brownie batter. Baking a few walnuts on top of the brownie batter gives them a nice toasted flavor and makes the brownies pretty.
Bake the brownies in the wood fired oven for 30 minutes at 350 degrees Fahrenheit. Alternatively you can cover the pan with aluminum foil and bake them on top of a wood burning stove.
When the timer goes off, remove the pan from the oven or off of the top of the wood burning stove. Insert a toothpick into the center of the brownies to make sure they are done. It should come out clean – or nearly clean. If you notice a lot of chocolate batter on it, return the brownies to the heat source and cook them slightly longer. Be sure to check them about every three minutes so you don’t burn them.
Once the brownies are done, allow them to cool for 15 minutes, then remove them from the pan.
Serve immediately or allow them to completely cool and then cover them tightly with plastic wrap.