Preheat the oven to 180C/350F/gas 4. Grease a 20cm/8in square baking tin with butter, then line with baking paper with the paper overlapping the sides a little.
Melt the butter in a pan. When it has melted, remove from heat and add chocolate. Leave to melt, then stir together.
Whisk the eggs, egg yolks and vanilla together in a large bowl until the eggs begin to get light and fluffy. Add the sugar in 2 additions, whisking between each. Pour it around the side of the egg mix so as not to knock out the air that has been whisked inches Keep whisking until the mixture becomes stiffer. Once the egg mixture is ready, pour the chocolate into it - again around the sides so as not to knock the air out.
Add the flour, cocoa and pinch of salt and a third of the biscuits until fully combined, then pour the mixture into the prepared tin. Scatter the remaining biscuits over the top, pressing them in slightly. Bake on the middle shelf of the oven for 25-30 minutes. The middle should be very so slightly gooey. Leave the brownies to cool in the tin - the top will sink and crack a little.
Pull the brownies out using the overlapping paper and cut into squares. Dust with icing sugar.