Preheat your oven to 170C / 325F / Gas mark 3.
Place your butter and chopped chocolate into your skillet and gently melt it over a very low heat.
Once melted remove your skillet from the heat and add in your sugars and mix well.
Add in your eggs one at a time mixing between each addition. Then mix in your vanilla extract.
Add in the flour, baking powder and salt and stir to mix everything together.
Clean down the edges of your skillet with a rubber spatular and push the batter in towards the middle of your skillet.
Place your skillet in the middle of your oven and allow your brownie to bake for at least 50 minutes.
Once the time is up take your skillet out of the oven but leave the brownie alone for 15 minutes to slightly cool down and firm up a bit more.
Cut up and serve if you wish to eat it warm. If not allow it to fully cool down before cutting it into wedges and storing it.
This brownie will keep well in an airtight container at room temperature for at least 5 days.