Preheat your oven to 180°C(160°C fan assisted) and line a 9x9inch square tin with baking paper.
While the oven's heating up, melt the chocolate and butter together. Do this by placing a heatproof bowl over a pan of simmering water. Don't let the bottom of the bowl touch the water and stir until the butter and chocolate have melted together.
Leave this mixture somewhere cool a little while you do the rest.
In the bowl of an electric stand mixer, or using an electric whisk, beat together the eggs, egg white and sugar. This will take a while, so a stand mixer will be best. Whisk for 7-10 minutes. The mixture will have gone pale and yellow. If you pick some up on the whisk and dribble it across the top, it should take a couple of seconds for the line to disappear. This step is essential for getting the properly crispy meringue-like top.
Stir through the miso and the vanilla.
Carefully stir through the chocolate and butter mixture, making sure you don't knock much air out.
Carefully stir through the flour, again making sure you don't knock the air out.
Transfer the mixture to the prepared tin, and place in the middle shelf of your preheated oven.
Bake for 17-22 minutes. You want the top to have crisped up and for there to be just a little jiggle when you wobble the tray.
Place the tin on a cooling rack and allow to cool completely, and set, before serving.