Preheat the oven to 170°C/330°F/Gas mark 3. Grease 2 baking trays or line with parchment.
Sift together the flour, baking soda and salt into a bowl then set aside.
In the other bowl, cream together the melted butter, brown sugar and caster sugar until well blended. Beat in the vanilla, egg and egg yolk until light and creamy. Mix in the sifted ingredients until just blended. Stir in the chocolate chips by hand using a wooden spoon. Drop cookie dough onto the prepared baking trays, with each cookie around 4 tablespoons of dough. Do not flatten the dough. Cookies should be about 8cm apart.
Bake for 15-17 minutes in the preheated oven, or until the edges are lightly toasted. Cool on baking trays for a few minutes before transferring to wire racks to cool completely.