Preheat the oven to 180C & grease & line a 9x9inch square baking tin with parchment paper.
In a heatproof bowl set over a pot of simmering water, melt the chocolate & butter together.
While the chocolate mixture melts down, whisk together the eggs, egg yolk, & sugar in a large bowl until thoroughly combined & pale in colour.
Pour the melted chocolate into the eggs mixture & stir together with a whisk.
Using a spatula, fold the flour into the mixture & pour into the prepared baking tin.
Bake in the preheated oven for 25-30 minutes, until the top is papery & the centre is still gooey with a slight wibble.
Leave the brownies to cool completely in the tin.