The day before you want to bake/eat the cookies, melt 340g of the chocolate chips in a bowl over a saucepan half full of water and bring it to a simmer. Stir slowly with a spatula to ensure the chocolate chips are completely melted and smooth before turning off the heat.
Melt the butter in the microwave (about 30 seconds on high).
Mix it into the melted chocolate, keep the mixture warm over the hot water, it will be quite thick once you have mixed in the butter.
In another large bowl, mix the granulated sugar, cornflour, baking powder, and salt.
Add the eggs and whisk until fully incorporated and the mixture resembles a thick batter and no dry ingredients remain.
Slowly whisk in the melted chocolate–butter mixture. (If it has cooled and begun to solidify, gently reheat it before incorporating.)
Gently fold in the remaining chocolate chips and the pecans with a spatula.
Transfer the dough to a baking dish and cover it tightly with cling-film that touches the surface of the dough.
Place in the fridge to set overnight.
The next day preheat the oven to 175°C Fan/190C and set a rack in the middle of the oven.
Line two baking sheets with baking paper.
Using your hands, break the dough into pieces that weigh 50g, roll the dough into a ball and place it on the baking sheet, repeat with the remaining dough. Place them at least 2 inches apart from one another.
Press gently on the top of each ball with the palm of your hand to make a thick disk. This dough doesn’t spread much, so the cookie should be relatively close to the size of cookie you’d like.
Bake for 4 minutes on the middle rack, rotating the baking sheet 180 degrees halfway through and bake for a further 4 minutes.
When the cookies are just beginning to crack on top and the dough is set on the edge and has a soft spot in the centre (about the size of a 50p), remove from the oven.
Place the other tray of cookies in the oven and cook for the same cooking times as above.
Let the cookies cool on the baking sheet for 5 to 7 minutes, to further set. Remove the cookies from the baking sheet and place on a wire rack to cool.