Preheat the oven to 180 C (350 F). Grease and line an 8 inch square pan with parchment paper (20 cm pan)
Melt the chocolate and butter together. Stir to combine and set aside 5 minutes to cool
Add the eggs, dark and light brown sugar and vanilla to the slightly cooled chocolate and mix well.
Whisk together the flour, baking powder, salt and cocoa powder.
Add the flour mix to the chocolate mix and stir until just combined.
Pour half the batter into the prepared pan. Top with a layer of dulce de leche, as thick as you like, then add the remaining batter and spread carefully to cover. Sprinkle with sea salt if desired.
Bake for around 30 minutes or until toothpick inserted in the center comes out with some moist crumbs. Don't overbake!
Notes
Method for making the dulce de leche and melting the chocolate detailed in the blog post above.