Cut the butter into cubes, and add to a heatproof bowl set over a pot of simmering water.
Leave to melt, then remove from the heat and allow it to cool down.
Add the cooled butter to a large bowl, and add the white and brown sugar.
Use a hand mixer to mix everything together until you get a smooth paste.
Add the egg and egg yolk, and mix again.
In go the sifted flour, bicab of soda, vanilla extract, and mix again.
Throw the chocolate chips in, then use a spatula to mix the ingredients together.
Leave the mixture to rest for 5-10 minutes so that the dough can thicken a bit, it's pretty soft at this stage.
Line a large baking tray with non-stick baking paper.
Preheat the oven to 180 degrees Celsius (350 Fahrenheit), and use an ice cream scoop or a spoon to scoop out small balls that are far enough from each other on the tray, I only had 6 on mine at a time.
Bake for about 10-11 minutes, or until the edges are slightly darker, and the cookies are still a bit puffed up.
Leave to cool on a hard surface.
Repeat with the remaining cookie dough, I baked mine in 3 batches.