Preheat the oven to 180°C / 350°F / gas 4. Break up the dark chocolate in to a bowl along with the unsalted butter. Put some boiling water into a saucepan on a low heat and rest the bowl on top. Gently melt the contents, stirring with a wooden spoon until mixture has melted.
Put the sugar and eggs into a mixing bowl and beat with an electric mixer for 2 minutes until thoroughly combined and the mixture is frothy.
Pour the melted chocolate into the sugar and eggs and mix well until you can't see any streaks of dark brown, this should take about a minute.
Sift the flour and cocoa into the bowl. Gently stir into the chocolate mixture with a wooden spoon. Add the vanilla extract.
Put the mixture into the prepared lined tin, I use a 20.5 x 30.5cm. Spread it out evenly, making sure it goes into the corners. Gently bang the tin on the work top to knock out the air. Carefully place the in the oven and bake for 20 minutes.
While your brownies are cooking prepare your vanilla frosting. Using your electric mixer combine the icing sugar and unsalted butter until it forms a paste.
Scrape the vanilla seeds out of your pod and add to your mixture, mix well.
Take the brownies out of the oven and put a skewer in - if it comes out clean then they're done. Let the brownies completely cool before putting a layer of frosting on.
Sprinkle with demerara sugar and using a blowtorch lightly caramelise. If you do not have a blowtorch then put under a very hot grill for a minute or two.