Preheat the oven to 180℃ (160℃ fan)/350°F/gas mark 4.

Prepare a baking sheet(s). Line a large baking sheet with greaseproof paper.

Melt the chocolate and butter. Break
up the chocolate and place into a large heat proof bowl. Cube the
butter and add to the chocolate.  Build a bain marie by setting the bowl
over a pan of water.  The water level needs to be shallow enough so
that it doesn’t touch the base of the bowl. Set the bain marie on the
hob over a medium heat.  Allow the chocolate and butter to gently melt.
Stir with a spatula regularly.  Once completely melted, remove the bowl
from the pan and sit onto a cloth for a couple of minutes.

Make the cookie dough. Lightly beat
the two eggs and egg yolk together in a cup.   Add to the melted chocolate mixture with the sugars and vanilla extract.   Stir
well with a wooden spoon until combined ensuring there are no lumps of sugar.  Sieve the flour, cocoa powder andcornflour into the batter.  Mix together well, checking the
bottom of the mixture to ensure there are no pockets of flour.  Leave
the batter for 2 or 3 minutes to allow it to firm up a slightly.

Shape the cookie dough.   Use a spoon to
create balls of dough, of roughly equal size.  Each will weigh between 50g – 52g and will have a shiney appearance when rolled between the palm of the hands to create a neat ball.  Place onto the prepared baking sheet, spacing them well apart to allow for spreading during the bake (our 30cm square bakng sheet held 5 cookies).   Slightly
flatten the cookie dough balls with your hand to create thick discs.

Bake. Place the baking sheet into the
oven and bake for 13 minutes, rotating after about 9 minutes of baking.  The cookies will have puffed up slightly and will be
soft at this stage.  Allow the cookies to cool for 5 minutes on the
baking sheet. By this point the cookies will have flattened and will be
starting to firm up. Use a pallet knife or fish slice to carefully
transfer them to a wire rack to finish cooling and firming up.

Continue shaping and baking the cookie dough.
Depending upon the size of your oven, it’s likely that you will need
to batch bake the cookies. You may also need to change the greaseproof
paper if reusing the same tray.