Preheat the oven to 180 degrees C and grease and line an 8 x 8 inch pan.
Break the chocolate into a heatproof bowl and add in the butter. Place in the microwave in 20 second increments stirring between everyone until everything is melted. (Make sure you stir for a good 30 seconds because chocolate can sometimes be melted, but still hold it's shape so if you stir a few lumps, you might not need to heat it again!) If you're not comfortable melting chocolate in the microwave then place the heatproof bowl over a small saucepan of simmering water and stir until melted.
Leave to one side to cool. Place the sugar, eggs and vanilla extract into a large bowl or the bowl of your stand mixer and mix until smooth and well combined, about a minute on med-high speed.
Sift the flour, cocoa, baking powder and salt together into a medium bowl.
Add the melted chocolate to the sugar and egg mixture and mix until well combined and smooth.
Tip in the dry ingredients and fold in on a low speed until well combined.
Add in the dulce de leche and stir by hand, don't incorporate entirely, leave lots of glorious swirls :)
Pour the mixture into your baking pan and smooth over, being careful not to undo your work of making the swirls!
Place in the oven for 25-30 minutes until an inserted skewer comes out with a few crumbs still stuck to it.
Leave to cool completely in the tin.
Once cooled slice into 9-12 bars and store in an airtight container at room temperature for 4 days.