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.
Break 300g chocolate into pieces and place in to a large heat proof bowl with 125g butter cut into cubes. Place the bowl
over a pan of gently simmering water. The water level needs to be shallow enough so
that it doesn’t touch the base of the bowl. Stir until the chocolate and butter have melted and combined. Once completely melted, remove the bowl
from the pan and sit onto a cloth for a couple of minutes.
Lightly beat two eggs and one egg yolk together in a cup. Stir 100g granulated sugar and 125g muscovado sugar into the melted chocolate mixture. Then beat in one teaspoon of vanilla extract and the beaten eggs. Stir
well with a wooden spoon until combined, ensuring there are no lumps of sugar.
Sieve 225g flour, 40g cocoa powder and 1 teaspoon cornflour into the batter. Add 75g chocolate chips. 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 slightly.
Use a spoon to
create balls of dough, of roughly equal size and roll into a ball. Each ball should weigh about 50g (2oz) and will have a shiny appearance when rolled between the palms of the hands.
Place onto the prepared baking sheet, spacing them well apart to allow for spreading during the baking. Slightly
flatten the cookie dough balls with your hand to create thick discs.
Bake for about 12 minutes, rotating after about 9 minutes of baking. The cookies will have puffed up slightly and will be
soft at this stage.
Remove from the oven, then 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 completely.
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.