Preheat the oven to 160℃ and brush a 5 inch cast iron skillet with a little butter or oil.
In a bowl add the chocolate and butter, warm in the microwave for 10 seconds intervals until melted. Took me 20 seconds.
In another bowl whisk the yogurt and sugar until the sugar has dissolved - this should take 3-5 minutes.
Add the cooled melted chocolate mixture into the yogurt mixture and whisk until combined.
Add the flour, cocoa powder, baking soda, baking powder, salt and whisk until smooth - if your dry ingredients have lumps in them - pass it through a strainer before adding into the batter.
Pour the batter into the prepared tin and top with chocolate chunks, bake for 18-20 mins. If you like the brownies more gooey then bake it for less time. I would always recommend under baking the brownies. You know the brownies are done with the top has risen, is shiny and has a crust.