Preheat oven to 175°. Line a 30cm x 25cm, deep cake tin with baking paper. Melt dark chocolate and unsalted butter together in a microwave on low heat. Heat in bursts of 30 seconds, stirring in-between until smooth and melted.
Beat eggs and caster sugar together until pale and creamy. Stir in the chocolate mixture and vanilla essence. Sift in the flour and cocoa powder, and stir to combine.
Fold in the chopped white chocolate and pour into the prepared tin. Sprinkle over the sea salt and bake for 40-45 minutes until cooked through. Cool in the tin before cutting into 5cm squares.
