Preheat oven to 180°C.
In a bowl add chocolate and butter.
MW at 10 second intervals until they both melt to form a shiny chocolate pool.
Add sugar and whisk until completely incorporated.
Add cocoa powder and whisk until combined.
Add in vanilla and whisk.
Add one egg at a time and whisk until completely incorporated before adding the next.
After all the four eggs are added and whisked well, add in the flour and using a spatula, fold it with the chocolate to form a thick batter.
Grease and line an 8" square tin and pour half the batter into it.
Add half the dulce de leche in dollops and using a stick or a knife create swirls.
Add the remaining batter on top and add more dollops of dulce de leche.
Again create swirls.
Bake in the preheated oven for 25 - 30 minutes or until the brownie is firm.
Allow it to cool completely.
Cover in cling film and refrigerate overnight.
The next day, remove from the pan, slice it up and enjoy!