Preheat the oven to 180C/350F and spray an 8x8 inch square pan with cooking spray. Lay a long piece of grease proof paper (around 4 inches wide) across the pan, and repeat going the other way so you have overhang of grease proof on each side of the pan to act as "handles".
Break the dark chocolate into small pieces and place into a heatproof bowl with the butter. Place in the microwave in 20 second increments, stirring between each one, until smooth and completely melted.
Alternatively you can place the bowl over a pan of simmering water and stir until melted. Leave to one side.
Place sugar, eggs and vanilla extract into a large bowl and whisk by hand until smooth and the mixture starts to froth slightly.
Gradually pour in the melted chocolate mixture, whisking as you go until well combined.
Add in the flour, cocoa powder and baking powder. Switch to a wooden spoon/spatula and gently stir the mixture until well combined and there are no lumps of flour.
Place the caramel and salt flake into a small heat proof bowl and stir. Microwave for 10 seconds to loosen up.
Pour half of the brownie batter into your prepared pan and spread into an even layer that covers the bottom of the pan.
Spoon the salted caramel mixture on top of the brownies and very gently coax the caramel with the back of a spoon, to cover most of the brownies. Leave a 1/2 inch gap around the edge, but otherwise try and cover all the brownie batter.
Spoon the remaining brownie batter over the caramel and again, gently spread to cover the caramel. Go to the edge with mixture, but you'll have to be careful so you don't mix everything together!
Place in the oven for 23-28 minutes until the top is crisp and an inserted skewer into the centre comes out with a few crumbs of brownie. You'll need to be careful that you hit the brownie and not just the caramel. If the batter is done, you'll have a few crumbs, a gap where the caramel is liquid and then a few more crumbs where you hit the brownie.
Leave to cool in the pan for 30 minutes, before transferring to a wire rack to cool completely.
Once cool, slice into 9-12 bars and devour.
Salted Caramel Brownies will keep in an airtight container, at room temperature for 5 days.