(click on the text to highlight each stage):

1. Preheat the oven to 170°C/fan 150°C/325°F/gas mark 3. Grease two baking trays with extra butter or line with baking paper.
2. Sift the flour, bicarbonate of soda and salt into a large bowl. Set aside.
3. Cream the melted butter, brown sugar and caster sugar together in a medium bowl, using an electric hand whisk, until well blended. Beat in the vanilla, egg and egg yolk until light and creamy. Add to the sifted ingredients and mix until just blended. Stir in the chocolate chips by hand, using a wooden spoon.
4. Drop the dough on to the prepared baking trays - each cookie should be around 2 tablespoons of dough. Do not flatten the dough, as the cookies will do that by themselves, so leave a little space around each of them to spread.
5. Bake in the pre-heated oven for 15-17 minutes or until the edges are lightly golden. Cool on the baking trays for a few minutes before transferring to wire racks to cool completely.